MM8 is a smaller game, very much after the style of 6 & 7. I actually preferred it to MM7 because the inability for a party to have both Light & Dark magic in MM7 was a total turn-off for me.

MM9 uses a different game engine, and has quite a different feel to it than the previous 3 games. Opinions are divided, some like it, some loathe it. However, I think most would agree that if the team had been given a decent amount of time to develop it, it would have been a good game. Unfortunately, 3DO had to rush it out, and it was very buggy when it hit the market. Since then, a fair amount of work has been done, notably by Bones and others, to cure the more obvious flaws, and it is now a better game than it was. I can say that I enjoyed it (the combat system IMO is excellent), but there's no guarantee that you will.