I'm going to say KF1 is currently better, but KF2 is worth it.
Killing Floor 1 currently has more content (maps, guns, perks, DLC), its gameplay is fairly polished, it runs on worse rigs, and it's cheaper. It invariably goes on sale for dirt cheap during Halloween (AKA less than three weeks from now), because of the Halloween Sideshow event.
Killing Floor 2 has sexy amounts of polish, will eventually add in more perks than KF1 has for free, the shooting, hitboxes, gore, lighting, and map design is generally a lot more satisfying, and the game is actively being updated, unlike KF1 where I doubt it'll get any more updates except for major exploits or compatibility patches.
Problem with KF2 is that it's currently a diamond in the rough -- since the game was released, the community's been complaining that there's too little ammo compared to how many zeds there are per wave and that zeds generally have more health (compared to KF1, you only have something like 66% ammo to face 133% of zombies), Scrakes (the second-strongest non-boss zombies) are abysmally powerful, and some perks are hugely overpowered (Medic, Berserker) while some are nearly useless (Demolitions, Commando). Too much running around with no ammo getting chased by at least one raging mini-boss for seven minutes while you find the scarce ammo drops, then you get killed in two swipes after dumping 60% of your ammo into one of them and they don't die. The community keeps clinging to the idea that implementing the rest of the perks (mostly Sharpshooter) and TWI will actually make changes to the game for the better.
So far the development's been slow and selectively deaf to the community's biggest complaints. Since April there's been only one major content patch, one or two small additions where they added a map, and a few balance changes which seem to be be half content changes for the better (revamping Berserker, voting) and half content changes for the worse (removing early game weapon/ammo drops).
I will never deny that KF2 is satisfying, I will never deny that I really, really want KF2 to be good, I do think TWI listens to our criticism -- I just can't recommend KF2 when I think KF1 currently functions better with its content, and the tide of change with KF2 is so slow that a lot of people have lost interest on what is essentially a smaller version of a game that's been out since 2009.
Look back at it in a few months, maybe in spring, then ask if any of the issues have been ironed out.