FFF - perhaps I'm being a bit thick here, but I don't understand what the difference is.
If I get a Mode S transponder, the TCAS of an airliner coming the other way is able to chat to my Mode S transponder and find out what level I'm at, the same as Mode C. I can't see where the negotiation is going to come in, though, because I won't have TCAS. Sure, if both a/c have TCAS one can command a climb and the other a descent, but as I don't have it there's no negotiation to be done - my mode S transponder, if it's being honest, is just gonna shrug its shoulders.
Or am I missing something here?