szablev: Both have the Arcomage minigame, so you can't go wrong with either of them :D
On a more serious note, I have only played 6-7, so I can't give you a real in-depth comparison, but I think the most significant difference between the two is that in MM7 you generate your whole party yourself, and you get to choose from standard fantasy races: humans, elves, dwarves, goblins. In MM8 you only generate one character at the start and then you can recruit four more members during the game. However, MM8 gives you a more "exotic" choice when it comes to races, as you can have minotaurs, vampires, trolls, dragons and the like in your party.
Since I haven't played 8, I don't know if there are any changes to how classes, character development, spells etc. work, so hopefully someone else will be able to give you proper information about that. Both games use the same sort of "semi turn-based" battle system introduced in MM6, which means that by default battles occur in real time, but you can switch to turn-based combat whenever you want to.
ah, thanks for the info :) And cool on the turn based combat! I have difficulty playing these types of games in a realtime/semi-realtime system.