I used to stick to King but it was too easy. Emperor is hard, and the computers definitely build wonders faster and seem to gain technologies quicker; but honestly once you have a good grasp of the game they need these handicaps just to compete with you.
I'd say if you've beat Prince a few times at least try King out. Emperor is when I find the computer actually feels like they're cheating.
"OH YA! Look at ya! Talkin all dat good **** a second ago! Then you got KICKED IN YO CHEST!" -Col. Stinkmeaner