The reason people argue that Fo3 has better exploration is because it's designed, as people often point out as a criticism, like a theme park. That's how Bethesda go about making games, the same thing can be seen in Oblivion and Skyrim. If you start walking in any direction in Fallout 3, the screen will be filled with all kinds of ridiculous shit, whether its a Super Mutant Behemoth guarding a teddy bear, one of those skeleton-filled houses that Tim Cain spunked over, the stupid Lovecraft building etc. To someone looking to play a pure action-adventure game (in other words, most of Fallout 3's fans) this is a good thing, because you're never more than 5 minutes away from a nuclear-exploding car or whatever.
New Vegas is more grounded in coherency and at least semi-logical worldbuilding, meaning that if you start walking in any given direction in New Vegas, you'll come across towns, ruined wasteland, NCR camps, raider outposts and so on. There won't be any mega-cool nuclear car blast chain reactions, no giant Super Mutants locked inexplicably in cages waiting for you to free them so they can go apeshit and destroy everything, no glitchy Lovecraftian buildings, no caves filled with little kids who say "fuck", no wicked-sick-uber-awesome hidden weapons that let you blast people's heads off by firing teddy bears at them etc.
So if you're someone who doesn't give a shit about logical worldbuilding, coherency in the setting and creating a believable world but does want to see Todd's coolest and raddest ideas, Fallout 3 provides all the action thrills and weird hidden shit you're looking for. To those people, Fallout 3 does have better exploration for that reason. To people who are interested in lore and all the stuff standard Fallout 3 fans aren't interested in, exploring the map of New Vegas and piecing together how everything works is much more rewarding. I don't even mean that as a criticism of Fallout 3 or it's fans necessarily, I think it is actually genuinely entertaining to see what kind of weird shit ends up in Bethesda games.
EDIT: Managed to post an unfinished message