Oblivion is an amazing game, but it's fairly dated these days. If you're going to pick up Oblivion, get it for the PC. For a four year old game, it looks pretty damn amazing with the right mods. You can pick up the GotY edition on Steam for $25.
Dragon Age is also an amazing game. Sadly, it's also much better on the PC than the 360. It is better balanced, has better graphics, and offers a superior range of views. My wife played it on the PS3, though, and thoroughly enjoyed it.
Honestly, they're both excellent titles, and I'm sure you'll enjoy either of them. If you're looking for a cheap game that is ridiculously open-ended, go with Oblivion. If you don't mind spending a little more and you prefer a rich but relatively linear storyline, Dragon Age is the way to go.