To play online I'm almost positive you need to join the RSSC, so yes everyone is playing on 1 platform. I'm pretty sure that if they were two separate services that we would have heard about it by now. I still don't see myself doing much with that. Given the fact that I can't wait to MOD the game I wouldn't be surprised if I got banned for "cheating" even though I didn't do anything.
Update: Another note about the GMG sale is that you get a free game with each purchase. Of course YMMV and odds are you're not going to get a AAA release from the past 3 years for free. I ended up getting
this game that's just 2 months old, although it is part of a franchise apparently. Might record a video about it tonight.