OP has probably found his answer by now and made his purchase but...
Fallout 3 in my personal opinion is a much better game. It's very fun to play and has many qualities similar to The Elder Scrolls: Skyrim. There's a quality to the world, the characters you meet and the adventures you have. It may not be by the guys who made the old games, but that is a poor standard to judge a game by, who makes it. If the work is good -- it is good, it doesn't matter who made it.
A lot of people don't even try FO3 and just play NV -- indeed many in this very thread have suggested that you do just that -- but in my opinion, this is because of fan-loyalty and personal bias. The fallout series was a sore spot for its core fan base for a long time, because the 'real' Fallout 3 never got to see the light of a release. That's why many people claim that the actual Fallout 3 made by Bethesda Softworks, has "terrible writing" and that you should play New Vegas made by the "old Fallout team" in reality, Fallout New Vegas wasn't entirely made by the same exact team as the old games, and it shows many missing qualities. Not to mention that it borrows extensively from Fallout 3's engine itself. It seems silly to not even give FO3 a chance.
Anyway, now to my personal opinion on New Vegas.. I didn't like it. It's for a start, the first Fallout game to not have you start out as a vault dweller or have atleast some relation to vaults.. you're just some average joe. Now, in my opinion that right there is poor writing. Not giving the fans what they expect -- or wanted for many years. A game that goes alongside what we loved in the old games. NV changes all that and turns the Fallout world into a wild west cowboy romp with enemies from the Fallout universe just lazily tossed in and randomly placed about. You'll fight end game enemies right outside the starting town for the beginning of the game. You'll find invisible barriers that never existed in the game series before, you'll see a lack of play testing in the gameplay that's readily apparent and you'll encounter a lot of frustrating bugs that pin you into a single type of gameplay. Using guns. I encountered repeated crashes and freezing issues when using melee and unarmed special attacks (playing on a console, no less! the easiest system to rid of bugs!) that were there in October 2010 and still exist. Buyer beware... they haven't so much as touched the game since it's release, it remains a very glitchy game. One more complaint I have about the game is every, single, person, you meet, is bland and monotone. "Like a cowboy", they all talk in the same accent and they all have no real personalities. The only characters that stand out are big name actors that were paid to voice dialogue for certain characters, but this doesn't happen until very late in the game (if you excuse the beginning cutscene) and it doesn't really feel that special after talking to 205 boring monotone characters that repeat the same boring lines "Patrolling the Mojave almost makes you wish for a Nuclear Winter". Go ahead, youtube that line to get an idea of what I mean.
Wall of text, awayyyy!
Them's my opinions. Take 'em with a grain of salt... but know that I love the Fallout series, but not to the point that it blinds me.