I'd say it largely depends on what role you want it to fill. The Deredeo is a ranged attacker with a potential bit of buffing for other units in your gunline. It's flat out a long ranged heavy fire support platform, and it's very good at the role - it's especially good for shooting Jetbikes and heavier Jump infantry. The only compromise is the weirdly specced Invulnerable save, which I suppose might let friendly units get clear from the explosion if it gets into melee. It can do excellently if you're aiming for a defensive posture or to support squads all over the field from a safe-ish distance.
The Leviathan is quite a bit tougher, especially if your enemy doesn't have easy access to S9+ weaponry (speaking as a Sisters main, the amount of Meltaguns it takes to down one of the things is ridiculous) with the point cost to match. While it's got good ranged weapon options, they're all fairly short ranged. This thing is as subtle as a brick to the face, very much a Dreadnought meant for supporting an aggressive push, getting stuck in and ruining someone's day - though it can get swamped pretty easily on its own. Note the enthusiastic 9" D3 explosion.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.