Simply put...
300C: Carburated, engine not derated, 3 seater so FP in left seat.
300CB: Carburated, derated engine (higher TBO but less powerful), 2 seater, FP in right seat. Was designed to be a trainer and attract schools back to the Schweizer brand after Robinson started invading the market.
300CBi: Same as the CB except it is injected.
Flew the CB and C and they are great little things... if only they could be a little faster!!!
Might be more subtle differences but that is the big stuff. Hope it helps!
G