Very easy answer actually, Get the cheaper Fallout 3 on PC and then go to one of the many mod sites and spend a day or so looking for some of the better mods and you will end up with a game that does everything FONV does but in a better setting. New Vegas is made out to be some big goal on your overall journey but the fact is once you reach both it and the town of Freeide it is a massive let down the rest of the map is no more or less interesting than what appears in FO3 the bonus being that the DLCs for FO3 give it a big edge over NV.
Or you could just skip all that and get Oblivion which owns both of them.