I'd say both, but I chose NV just since I've been playing it lately. You can choose to either "prepare" for Skyrim or just have some good ol' wasteland fun. I know you say you're indecisive, but you must have a mood one way or the other at least a tiiiny bit.
Actually, I'm still entirely indecisive on which game to play. At this point in time, my thoughts on each game are equally 50% to 50%. I'm using this thread to push the balance to 49.999% to 50.001%. Right now it seems like I'm being pushed more towards New Vegas in the comments, but Oblivion in the poll, which I have no problem with, but it still keeps me at an even 50%-50%. I'm planning on making my decision on Monday afternoon.
Decisions like these are important to me, because I generally spend a lot of my free time playing video games, and I'm the type of person that usually focuses on just one game.