DA:O is better for combat and plotline; it's a typical Bioware game, so it's very plot oriented, and has a ton of storyline to go through that'll keep you going for hours. The combat can look a little bit boring, but it's surprisingly tactical and definitely enjoyable to work out what works best in what situation and everything.
Oblivion is better if you like exploration, sandbox games, more diversity, and so on. The open-world feature makes it feel huge, the diversity of quests and options gives you more to do than in DA, and even if you don't want to do any quests just exploring makes things fun. Top that off with nearly unlimited ways to get money, increase your character, and so on and it just seems like a bigger game that has more to do in it than Dragon Age. Plus the mods for it are ridiculous.
Dragon Age 2... Well, my suggestion would be to wait until it's in the bargain bin. It's worth a playthrough, and you might like it, but I thought it was horribly mediocre, rushed, and definitely not worth paying full price for. It just seems small and insignificant when compared to either of the others, and the gameplay is often a mating of the worst characteristics of either game.