The Leviathan is definitely harder to kill, with 1 more Armor, 5 more Life, Tough -3, and Hydro -3. It also has the spiffy Unstoppable trait, but I don't think that makes a huge difference on slow creatures. It actually makes it harder for it's controlling mage to put it in position because you also can't use pushes on it. Teleports are a must.
So what does the Kraken have in it's favor? Easy. Tentacle Grasp. Don't underestimate a ranged attack on a slow creature that can pull targets back into the zone when they attempt to escape. Much easier than chasing them around. And then the new grapple mechanic will keep them there. The Kraken is a lot easier to maneuver around and harder to get away from than the Leviathan and thus will be able to do damage much more consistently. If your opponent comes within one zone, you can grab them without using any other spells. Your opponent can probably just kite the Leviathan without too much trouble, but will have much more problems doing so against the Kraken.
Their main attacks are comparable. 7 Dice with Pierce +2 and 6 Dice with Pierce +3 ain't a big difference in my opinion. You might get on the fact that the Leviathan's is a Quick Action and can be used with guard, but again the Kraken can grapple a creature. Then that creature CAN'T attack anyone other than the Kraken, not even with sweeping attacks, and the creature suffers a -2 penalty to attacks made against the Kraken. The grapple also can't be "broken" by a tiny creature to let a big critter through. You choose the target and can make sure the biggest threat is neutralized. That's a much better guard in my opinion. And the quick action doesn't really matter when moving because they're both slow. Although, if you do give them Fast to counteract the slow, a Quick Action Ranged attack on the Kraken is still more desirable to me as that gives him AMAZING access to the board. Anything within two zones can be in his grasp, and elusive kiters really can't get away then.
So, you trade a bit of survivability, for much better control and more consistent damage.