I dont think, there is any major difference between the "toughness" of MTTs on different sites. Its much more important, what the size of the buyin is. The competition is going to be much tougher in a 55$ MTT than a 1,1$ MTT, which honestly should be very obvious and not need any explanation. The more important differences are in things like field size and structure. And here you basically need to just try the different options and see, what suits you best.
One general and important thing to consider is, that variance will increase with field size. This is one of the reasons, why I leaned towards
888 Poker for MTTs rather than PokerStars, but with the new MTT schedule on 888 Poker, the choise is no longer so clear for me, and currently I am actually playing on Stars again. The main reason for this however is, that I dont always have time for the big MTTs, so I have moved towards adding far more SnGs into my schedule, and here Stars just have way more game selection and activity.