3 and NV are the only ones I've played but theyre both phenomenal games. New Vegas poses more of a general challenge imo, mainly because you actually have to scavenge and its just more difficult in a good way. Plus there's always Hardcore Mode, and from what Ive seen the DLC is pretty kick ass. They took most of the things wrong in 3 and fixed it, especially with gun selection, mods and whatnot. They're both good anyway.