poker stars better software and more games
888 small number of people in tournaments, easier to win
imho
Pretty much this. I have played a lot of MTTs on
888 Poker, and I still think, its a great site for MTTs. Up to the 16,5$ level the rake is lower than on PokerStars, and the generally smaller fields reduce variance. 888 Poker have recently reduced the number of MTTs and made each one larger though, so the difference is not as big, as it used to be.
Recently I have been playing on PokerStars, mainly because I wanted to get more into SnGs. But I have also been playing some MTTs, and I have to say, that apart from the higher rake and sometimes overly large fields, PokerStars is a really good site for MTTs. There are far more games running, especially if you are a microstakes player, and the structures are generally very well thought out.
In top of that you can combine MTTs with some of the 18-180 man SnGs, which makes it even easier to get volume. As for the "toughness" of the games, I dont think, there are any major differences. On PokerStars the PKOs seem to be the most popular games among the recreational players, and as a result they are very soft up to at least the 4,4$ level.