I voted Epic. I have played at least one game on all speeds, and I prefer Epic well above any of the other options. It just seems more balanced. On normal it seems like any time I want to build a unit, I'm sitting there debating whether to wait 4 turns for the next tech that will make that unit obsolete. On Normal, every game seems to be a race to space.
At least with Epic, you have more of a choice of which way you want the game to go, instead of being pressured into one due to time restraints.
I probably wouldn't play any GOTM on other speeds, unless I was very bored. I just don't enjoy the other speeds nearly as much, but that's just personal preference.
Time played is more of a factor of how much you MM (or leave your comp on). I think my GOTM2 says 65+ hrs logged, but only was really about 5-10hrs. It's very possible to play quick games on Epic speed, unless you're just sitting there trying to milk every point you can.
Milking games is actually one of the reasons I decided to stop playing GOTM's back in C3, along with other nonsense like palace jumps and...ship chaining. Got tired of seeing all the "Best" players pulling these stunts in their reports.