Steam vs. Orgin

@Supermanslilsis  If you buy a Sims 3 pack in Steam, you can register the code in Origin and download it from your game library just as if you'd bought directly from Origin.  But it doesn't work the other way around: you can't register codes from Origin (or from discs) in Steam.  So if you want an expansion or stuff pack, it's more efficient to buy from Steam and then register the code in Origin.

Sims 3 Store worlds work differently.  They need to be installed through the store, and they will work in any kind of game install, as long as the account that purchases the world is the same one that installed the game.  So if your Steam base game code is in your Origin account, and you register the world to the store account tied to your Origin account (as in, you login with the same email address and password in Origin and on the store site), then you will also be able to download the world into your Steam install.  But if your Steam account has a different base game code than your Origin account, then the world won't work in both the Steam and Origin installs.