FO3 GOTY edition is well worth it in my opinion, but I love the bleak and harsh, grey atmosphere, the openness of the Capital Wasteland, ability to point and travel off in one direction and be able to avoid powerful critters at low levels if for example you wanted to make a trip to Republic of Dave soon after leaving Vault 101....
New Vegas feels like it has a big brother watching me & stops me from going in certain in directions by placing families of nasty criters in the way that are surrounded by long steep cliffs with no other routes through..... After wasting lots of hrs realising it wasn't really the same open game as FO3 I focussed on the main quest and then all of a sudden all the unique items started to appear and new characters got introduced to the game and lots of faction sidequests became available and the game has become very enjoyable and unique in it's own right perhaps.
I think it will be a few more months before I consider getting out FO3 again though, I've still got too much to check out in Vegas