Its personal choice. Fallout New Vegas is objectively the better game, from a roleplay perspective since your choices matter a bit more than just plain black and white, although Fallout 3's The Pitt DLC offers some very nice grey morality choices. Both are effectively open world games, though New Vegas discourages mindless exploring since you can run into things you arent capable of killing just yet, like Cazadors and Deathclaws which are very nasty early on, though become less of a threat the better equipment/level you have.
But the big difference are: New Vegas has a less linear mainquest, you can choose between 4 factions to support, Mr. House, the Legion, NCR or yourself. You have no subway system and as such there are significantly less ghouls, although there are still plenty of them. Less Super Mutants as well and no dumb "Vault with FEV" thing either.
So all in all, its up to you, Fallout 3 has some good DLC, such as The Pitt but has terrible railroad-mainplot and black/white moral choices. New Vegas is much more Fallout in the sense of the old games, where as 3 has more the feel of the war being over for maybe 10 years, and not 200. Honestly, now that there's a sale, just get both. But imho, and i played both extensively, New Vegas is the better one, but others might feel different.