I used to do the carrier back in the days but no more I get direct from Samsung like you said credit up front.EXACTLY. They won't even let you just buy the phone upfront after the trade-in credit. But it's because they know there's plenty of impatient people like me who will just say eff it, pay off early (losing the majority of that trade credit) and get a new device.
I've been losing my ace by doing this; that's why I decided to go the Samsung route. I get that entire credit upfront, no parsing out micro payments for a ridiculous length of time (3 YEARS).