Personally, I would go with Steam over Origin most of the time, except that Battlefield 3 is not released on Steam. Steam is simply more established and has a much larger library of games to choose from. Still, I haven't had any problems personally with Origin, so its really just a matter of how you want to purchase your games.
Origin brings an entire universe of EA games into a single, convenient application. It lets you purchase, download and play your favorite games, any time and any place you want. It also integrates social features such as profiles, friends lists and chat so you can effortlessly connect and play with your friends. It is a website - www.origin.com - where you can browse and pre-order games, as well as a desktop application that is the hub of your PC game-playing experience, from purchase to download to play.
Configure a custom computer with the Origin.