I suppose it's only expected that the Fanboys spawn all over this thread to defend the divine video game that is Halo... so to fuck them up, I'm going to do what they can't; back up my claims with evidence.
Inferior graphics to Resistance 2.
Despite what the Halotards claim, Resistance 2 has very good graphics. It also has a wonderful range of colours in it. No, that doesn't mean the player characters are bright red or bright blue, it means that the grass is green, sandstone is a bold red or strong orange, the San Francisco sunset looks like someone napalmed the sky and the 1950s automobiles are painted in vivid 1950s colours.
In fact, the only place, my dear 360 Fanboys, you will find "Real is Brown" is on the Chicago levels (which are all set at night, hence it's supposed to be dark), and the Iceland levels, which are all set inside a military complex. Military bunkers, as a rule, are not painted in bright, pastel colours.
Just because you can make things bright purple doesn't mean you should! Resistance 2 has picked an excellent mid-ground between realistic, and visually pleasing for its environments. Alas, dear people who wank off to Master Chief, looking at the Gears of War box is not going to tell you what Resistance 2's levels look like.
Inferior weaponry balance to Resistance 2.
How many people in Resistance 2's "godlike for sniping" maps have access to the Sniper Rifle? Everyone. How many people in Halo 3's "godlike for sniping" maps have access to the Sniper Rifle? Whoever spawns closest to it. Ergo, Halo 3 is inherently unbalanced.
Inferior weaponry in general to Resistance 2.
Resistance 2's weaponry all have near-perfect balance; every advantage has a disadvantage.
The three most-used Deathmatch weapons highlight this well.
Bullseye: Second weakest weapon, second-fastest rate of fire. Second-largest clip, up to 3 "Tag" shots as secondary fire (spawn with full Tag clip) that cause standard rounds to home in on the target. The Bullseye is excellent for close-range brawling, where lack of accuracy is not an issue. In theory, you can use the Tag-shots to 'snipe' with it, but the tags are just as accurate as a standard shot, meaning at mid-long range it's more about dumb luck than skill. It does happen, but you can't rely on it.
Wraith: Weakest weapon (shot for shot), fastest fire rate, largest clip (200 rounds, compared to the Bullseye's 60), Force Barrier that prevents damage as a secondary weapon, but deactivates when firing. Wraith also needs to "warm up" before firing.
The Wraith is a monster of a weapon when it gets going. However, there's a second or two delay, allowing an opponent to gun you down first. It's also very hard to move and fire, meaning you're vulnerable to flank attacks. Range is not great either.
Carbine: Third-weakest weapon, average clip, decent accuracy and rate of fire. Secondary weapon is a 1-shot Rifle Grenade (clip size 1; spawns unloaded). The Carbine is better at range than the Bullseye, and does more damage, but smaller clip and lower RoF means you cannot rely as much on "spray and pray" tactics.
In theory, the Wraith should always win head-on. The Bullseye should always win over the Carbine up-close (ie: close enough so it's shots don't "spread") and the Carbine should win outside of the Bullseye's optimum range. However, in practice the weapon that 'wins' is the one in the hands of the better player; a Wraith's Force Barrier is useless if your opponent stands behind you.
For every strength a weapon has, it has a weakness. You could argue that the Magnum is not 'balanced', in that it is in theory the worst weapon in Competitive mode, but every player spawns with it as their default Secondary weapon so that's a moot point; you'll always have a better gun to call upon. As such, the Magnum is intended to be used as a last resort, when your main gun runs out of ammo or just isn't suitable (the Fareye in particular, being a Sniper Rifle).
Smaller games than Resistance 2.
Put simply, anything you can play, I can play. The same is not true in reverse.
Both games could do a 2vs2, or a 12 player free for all. Can you do a 20-man Free for all? Can you do a 20 vs 20 Core Control [Capture the Flag]? You certainly can't do a 30 vs 30 Team Deathmatch / Skirmish game.
This is why more is better; the fact that if you want it, you can have it.
VASTLY inferior single player mode to Resistance 2.
Resistance's storyline overall is just better than Halos. It just is. Fine, Resistance 2 isn't as good as Fall of Man, but the same is true of Halo 2 and 3 compared to Combat Evolved; the fact remains that no matter how you want to play it, Resistance wins. If you want to judge them as they stand now, Halo 3's single player was a piss-poor tac on to keep fans happy, whilst Resistance 2's plot feels like it's there for a reason. If you want to judge the games on a "Day of launch" manner between Combat Evolved and Fall of Man, nothing about CE remotely impressed me. It's generic Space Marines shooting generic aliens. Fall of Man pissed off Christians around the world by having you blow up aliens inside Manchester Cathedral. Who didn't want Fall of Man after hearing that?
No Class-based gameplay, unlike Resistance 2.
Again, this is "we can do everything you can, you can't do everything we can." Competitive is not Class Based, but Co-Op is. This essentially gives Resistance 2 three distinct modes of play; Red-Ring-Of-Death superhero antics, Class-based team gameplay, and twitch-based frag-festing.
More bugs than Resistance 2.
Resistance 2 has sod-all graphical glitches, errors and so on. The Halo series in general is so famous for its glitches it practically became a sport to see how badly you could glitch yourself out of the level.
Less Character Customisation than Resistance 2.
In Halo, you get to choose between a Spartan, and an Elite.
In Resistance, you get to choose between Human or Chimera.
In Halo, you can choose your uniform colour, and your helmet.
In Resistance, you can choose your body type (of which both sides have quite a few), and your head/helmet, including specials such as Nathan Hale, a Cloven, etc. There are also some 'full body' outfits, such as the Kamikaze-creature for the Chimeran side, which change your entire outfit into a new creature type. Almost all bodies and heads can be combined. You can also add a variety of accessories, such as backpacks, canteens, radios, heat-sinks (for Chimera) and some truly crazy examples of headgear.
In team-games on Halo, you're pretty much stuck as a Red or a Blue; you lose the ability to customise your armour colour.
In team-games on Resistance, you're either Human or Chimera, but the options for that team remain totally unchanged.
So, now we've hammered those out, I want to apply yet another nail to Halo's coffin.
Halo Doesn't Encourage Teamwork!
Think about it; where's the incentive in Halo 3 to run around as a group? Do that, and some smart-arse with a grenade / missile launcher / Spartan Laser is just going to earn the "Blow up three n00bs at once" achievement.
In Resistance 2, one of the key abilities used in Team games are the Berserks. Some boost the individual, such as Iron Heart, Invisibility or the "Special Weapon" abilities, but others work on everyone nearby. For example, break out Ring of Life, and a shining circle of light appears. All friendlies in that circle, including you, heal at a much greater rate, to the extent they effectively have +50% / +100% health in a gunfight. Light that up on an objective, and you are bloody hard to shift. If people are toting grenades, you can always run off; the Ring of Life will remain for around a minute, so just scatter, rush in after the boom and kill the grenadier.
Alternatively, there are other options; Prototype Ammo gives everyone nearby more powerful weapons. Enhanced Radar makes it harder for enemies to sneak up on your little group. Hybrid Vision lets you and your hangers-on see through walls; especially deadly when armed with an Auger!
Ultimately, Resistance 2's Team-games cannot be won by a bunch of '3l33t' gamers doing their own thing; they'll get fucked by the middle-of-the-scoreboard squad who cover each other constantly, use complementary weapons and set up their Berserks to enhance their ability to work as a team.