speaking as a moderate player.. I experienced ladder burnout by the repetitive settings, well before half of the top players became top players...
I don't think I've played a game in 3 months now..
personally I think that MD > Rt just because of the ability to play people of higher quality with MD.. RT you can only play the people that are online *now* MD, you can set higher pre-req's and play better opponents.. and esp. in team games, MD will generally end up in you facing a better overall team then when your playing a RT game, and people are just rushing to get in
Although, I think my all-time favorite settings tend to include FFA RT, much for the reasons that Gui mentioned.. less time for collusion.. people can't think up all the angles and convince people of their positioning quick enough sometimes to team up against you, often not quick enough for it to still be relevant, as 2-3 turns after they start convincing, you could be big enough to be able to defeat them..
last favorite I remember..
played with 5, FFA