They're both great, though as a lot of people are saying, Fallout: New Vegas will simply last longer, and have more motivation to replay it again and again, assuming it can draw you in and keep you. Which is true.
However, Portal 2 is equally as good, despite not having that same replay-factor that New Vegas does, simply due to the humor, the clever plot, and the wonderful characters. Put simply, its better written than New Vegas, but it doesn't have the variety and freedom that the other does.
Since you were saying that you'll be playing on the 360, go ahead and ignore any comments about mods, map makers and the like. They won't do us console gamers any good. Overall, I'd suggest New Vegas all in all, since you get a little more longevity for what you pay.
But, if you're a sucker for good humor, great characters, and a brilliant plot? Go with Portal 2. It all depends on what you're, ha ha, aiming for.