@RohanBoda The way it's always worked, if you buy a pack on Steam, you also receive a CD key that you can then redeem in Origin, so all your content will be in your Origin library. (To buy packs in Steam, you also have to buy the Sims 3 base game in Steam, although it's also often discounted enough to make the total cost lower than merely buying through Origin.) However, Origin packs have never worked in a Steam install, so if you want to use content bought from both Origin and Steam at the same time, your only option is to play through Origin.
While the Steam terms of use haven't changed, it's possible that things might be different now that Origin and Steam accounts can be linked. If you linked your two accounts together, Steam might try to redirect you to the Origin version, although I don't know for sure whether this is the case.
The nice thing is that if you buy from Steam and can't register your codes in Origin, you can request a refund from Steam, as long as you've played the game for less than 2 hours total and it's been less than 14 days since the purchase. So there's no harm in experimenting.
