the Leviathan is noticeably harder to kill (and soften up with conditions) and can disable more creatures at a time.
I tend to dislike conversations about "well, if you do that, I can do this" because I do think there's an answer to everything in Mage Wars and these discussions are without point or end. However, I do think it may be useful to compare the Leviathan's restrain ability to the new Grapple mechanic, and some possible answers to both of them.
A big pro in the Leviathan's favor is that it's ability does affect multiple creatures whereas the Kraken's grapple only affects one creature. However, I wouldn't call these creatures "disabled" by the Leviathan's ability. They are merely Restrained. The creature grappled by the Kraken, however, becomes Restrained, Unmovable, can't attack other creatures besides the Kraken with non-spell attacks, can't use ranged non-spell attacks at all, and suffers a -2 penalty to it's melee attacks. A much greater disability.
Both of these effects can be countered with a teleport or banish. The Kraken's Grapple is also broken if the Kraken becomes incapacitated, and the prey gets a chance to escape after each of its actions (though the Kraken is likely to reestablish the Grapple afterwards). Meanwhile, the Leviathan's ability can be countered with Push effects. Not on the Leviathan of course, as he is Unstoppable, but on the victims. Restrained does not make them Unmovable. You can also completely eliminate the ability by replacing the terrain with one of your own.
Some interesting things about the Leviathan's ability. It still works if he's incapacitated, so a sleeping Leviathan will continue to restrain creatures. It also works against fliers, grabbing them right out of the sky and dragging them down into the depths. The Kraken can of course also grab fliers, but the passive nature of the Leviathan's ability means that no fliers will be traveling through his zone, as long as it has an Aquatic Terrain attached. Grapple can of course work on Mages though while the Leviathan's ability can not, and usually it's the enemy Mage that I'm really wanting to stop moving.
They are both very cool creatures imo. The Leviathan would definitely rock against a Spawnpoint if you can get him in the same zone and get some Aquatic terrain into that zone. Meanwhile the Kraken is a lot less dependent on proper positioning. They're both awesome.