Fallout 3 or New Vegas?

I see no one really answered your question, which is specifically about the final battles of the two games. Personally I'm going to have to go with the battle of Hoover Dam simply because of the story. To me Project Purity felt more like something out of cartoon, though Liberty Prime is hilarious and I love him, nothing about him or most of the battle with the Enclave in Fallout 3 felt like something believable. It was very "nyaaah we're the bad guys, and we're going to kill you!" President Eden was interesting, but that one conversation with him was literally the only interesting thing to me about the Enclave in the game. As for the actual fun of the battles in game, that's a hard one. Project Purity had Liberty Prime, who is as awesome as he is hilarious. But the end was really lame, Colonel Autumn sucked. Hoover Dam largely depends on what factions you sided with/helped. If you had most factions stay out of the fight it can end up pretty lame, but it can be pretty awesome if you didn't, like you said with the Brotherhood of Steel, the Enclave, the Boomers' air strikes, etc. I'm gonna have to go with Hoover Dam here as well because of all the different possibilities. I'm a sucker for factions. Also, Lanius is a hell of a final boss.

I would pick Fallout New Vegas, hands down. I is the newest Fall Out and probably more fun than the other ones. And if you don't download the patch there a money glitches you are able to do and it's quite fun :]

Idk man their both so good

Fallout 3 has a really good story but lacks in factions and more quest

Fallout NV has awesome faction missions and has more choices and the gun mods are cool but lacks in a good story and that emotion you get from fallout 3

Its hard to pick there both amazing games

It depends on what you like man. I have played both and traded them in because it is really hard and gets quite annoying because you have to repair your weapons and find lots of repair kits which are a pain to find. I would recommend buying either Skyrim or Oblivion both are great games. If you want to these games though I would go with Fallout New Vegas.

Save game and play both endings, thats how its done!