Spacemaster 2. I've been leaning toward RMFRP, though, just because I want to try out the newer system. RMX sounds like a nice, simple way to start, but after playing SM2 and running DnD with no real playing experience, I think I can probably handle a full system without too much trouble. I'm probably going to end up scrapping much of the setting and using strictly the rules, races, creatures, etc. when I build my campaign. Call me ambitious, but I just like to build my worlds from the core out. I figure it'd be more useful to go ahead and invest in a full system rather than a streamlined one, and the RMFRP core book seems to be the one book that actually has a complete base system rolled into it.
I might get supplements later, or I might bundle the pdfs, but I'm going to start simple. The campaign I want to build will be played "wake 'em up" style, meaning my characters will wake up not knowing who they are, where they are, why they're there, or even what system they're playing. I was introduced to SM2 this way, and it was actually a really fun and interesting way to learn the system. We didn't even get character sheets until just recently, after a few months of play. All we needed was dice and notebook paper, and it allowed a lot of room for exploration and character establishment without having to hash out the system. Right now I'm fresh out of cash, though, so this will have to wait 'till closer to Christmas to really get rolling. Even once I've got my campaign ready to play, there's no guaruntee I'll be able to get anyone to play it. If I can get this thing off the ground, I'll probably be posting here for tips fairly often.