Does the damage bonus on undead outweigh the damage that the Leviathan has over the CIV?
Sure, CIV hits less on the living, but if it hits harder than leviathan on undead that's pretty good too.
What are the pros and con to getting either?
I have a calibur with high bonus to slime. What tree should I go up?
Really, the bonus to undead isn't worth it, there are better weapons for dealing with the undead (Divine Avenger) than the CIV.
You're much better off maxing out that Damage to Slimes with the Leviathan Blade.
Go Leviathan!
My Ascended lvl 10 with High dmg bonus to Slimes hits higher than Faust in Tier 3.
I'm actually going cold iron. the damage isnt a huge different I'd think and undead are really the only mobs that I have troubles with (damn you phantoms). That, and the cold iron vanquisher looks FAR better than leviathan.
CIV looks way better than Levi, if you ask me. Even if CIV didn't have the bonus I would have went with it. Also, I've heard that CIV and Levi have the exact same damage by Lv10 (Levi just starts a bit higher). Ultimately, this would make CIV and Levi the same, but with CIV having the bonus of being much more effective on Undead-types. The only downside would be that it is a lot harder to find the recipes and materials for CIV than Levi.
Edit:
By the way, this is mostly based off of what I've heard as well as the fact that both of the Damage bars for these two weapons are identical in length. But honestly, this does not mean much considering, for example, that the Speed bars of both Faust and Avenger are completely different, yet they have the same actual attack speed. I think the Devs update equipment too often to remember to update the tooltips...
Leviathan:
Pros:
Better on absolutely everything but Undead.
Cons:
Worse damage on Undead.
For pros/cons of Cold Iron, just reverse them.
Basically, it comes down to what you find yourself fighting more often - Undead, or Everything But Undead? Eventually it's in your better interests to make one of each if you really do fight a lot of Undead. Personally, I only encounter one, maybe two rooms containing a large enough amount of undead per T1-3 run to warrant their own weapon.