classic valves can use CO2 while RT's can't. the RT's are capable of higher rates of fire and some are able to have a prominent reactivity. Classics normally can operate at up to about 13 bps
The trigger pull on a classic is greater than an RT. Not all classics can use the ULE trigger kit. Increasing the inout pressure to a classic valve doesn't change anything aside from velocity.
That's just off the top of my head. there's probably more...