I have all my Sims 3 games from Steam. And I actually like it. Never really had problems, except when I really wanted Supernatural, and it only showed up on Steam a lot later. But after that I think every new EP has been there on the release date (I'm not sure about Seasons, though).
Anyway, I would prefer using Steam, if I get TS4. But it seems likely it won't be on Steam, so if I want it at some point I will probably have to get it through Origin (don't see any point in buying discs). Now I have it installed anyway, because I wanted TS2 Ultimate Collection.
The only problem I've found with Steam is that it doesn't seem to have the Sims -games on sale as often as Origin. And like someone already mentioned that once you have the base game installed through Steam, you're tied to purchasing all the EP:s through Steam, too. But, since I like using it better than Origin, I don't really mind.
Oh, and then, I don't know if it's possible with Origin, but with Steam I get all the updates installed automatically as they appear, so I don't need to keep up with them. Steam takes care of it for me.