Lever guns look cooler yo.
Also I know I'm pretty late to this party, but overall I actually liked Fallout 3 more than Fallout New Vegas (though I like both a lot).
The ultimate selling point for me was enemy variety, and "dungeons". I feel like the dungeons in Fallout 3 were designed better each with it's own little story. While Fallout New Vegas TRIED to keep that, it fell flat on it's face so often. A perfect example is right in the starting area. In Fallout 3 you had the school that raiders were using to try and burrow underneath Megaton until they hit giant ants. Those were serious enemies at the level you were facing them, but ultimately beatable, and it certainly felt good to to have cleared everything out of there right after leaving the Vault.
In Fallout New Vegas, you have a school. . . with a few Giant Mantises (manti?). Those enemies are such a joke I run up to them with my broad machete and one shot them. Then I get my one chest of mediocre loot. OK, so the starting area is a bit silly, let's fast-forward to Primm. You invade a hotel that's held by convicts who have broken off from the main group of Powder Gangers. OK, most of them aren't that bad, OMG THE LEADER HAS A FRIKKIN' INCINERATOR!!! As a melee-centric character, I can say this encounter is pretty damn hard (almost impossible) without switchin' over to guns, and I really don't like to do that, or eatin' the med-x and stimpacks like a mo' fo'. Other than that it's pretty easy, get a nice number of guns, armor is pretty lacking, but what are you going to do?
Ultimately, that's the selling point for me. The dungeon type areas in Fallout 3 are of much higher quality than Fallout: New Vegas. I'll admit the overworld in NV is better, the radio is better in NV, the characterization of companions is better. ((I never actually used companions in Fallout 3, just never cared enough)) If it wasn't for the boring enemy variety and dungeons (I also liked the feat/skills and crafting better in FO3, but not enough to make a big deal out out of it) it'd be better all around. For me though, those issues alone push FO3 on top.