Some guys will of course say about 317 cc difference.. That is about it though. Same frame and lower end. I think the 1200 has a larger bore and stroke and a lot more torque and hp and an air cleaner that looks like a can of ham.
The bore is larger and the heads are larger on the 1200, the jetting is differant (if carberated), the gearing is lower on the 883, and the ignition is slightly differant.
The clutch in all Sportsters will easily support a 100 HP.