I don't usually play RTS', in general. The only one I really played more than an hour of was Warcraft 3, but even that wasn't able to maintain my interest for more than about a week, lol. I really enjoy RTS', actually, but almost in the same way I enjoy a game like GTA... ie., it's really fun to play every once in awhile, but I can't get completely engrossed in it for the long haul the way RTS fans can.
I never played Total Annihilation...
however, I have heard the soundtrack (I think it was one of the first game soundtracks I ever downloaded), which has always been one of my fav game scores ever.
The demo for Supreme Commander, unfortunately, wouldn't run for me. It started up, and then immediately cancelled out and told me to verify that I met the minimum system requirements, lol. It's probably not something that'll have been fixed in the retail version, my machine really is a piece of junk at this point. I seriously need to upgrade it. Luckily, my birthday's just around the corner.

After that, I'll be sure to give the demo another try, heh.