I'd say New Vegas is more in depth than FO3, the map might not seem quite as big as f3 but for me at least there's more to see and do, the ability to mod weapons, a more strategic use of supplies and expanded companion
options make it the better game.
The story was a tad stronger in Fallout 3, but as NEUROTIC HOG wrote there's much more to do in terms of those little things. My draw with FNV is that I can actually make things unlike the last one where you needed schematics & even then you could only
make a certain amount of things.