I guess it largely depends on what kinda game you wanna run...
Horror/high fatality/tentacled goodness?
or
Exotic/high fantasy/sentient-jellyfish-as-a-PC-race goodness?
Cthulu is great if you're not wanting to run a more standard D&D game. Heck, even if you are, it's fun to pick up and add to your campaign (my summer campaign had Cthulu overtones and was a blast...). It's not for the heroic player, but it is for the malicious DM....
Oathbound is just sweet. It largely defies genrea-ization, so I'll just describe it. Ancient god is sealed away someplace. His lackeys/fellow pantheon members are stationed around the someplace to stop him from ever coming out again, forced to take an oath with other gods to that effect. The lackeys, over millenia, find loopholes/gaps in the rules of the universe that allow them to, among other things, steal bits and peices of other realities for their own.
So the world is a great mix of a lot of different elements.
Hurm...I may even encourage the melding of the two. Drop some Oathboundites straight into something out of Lovecraft. See how eager they are to conquest, now...
Basically, I'd go with Oathbound unless your group has a high number of Lovecraft fans or you especially dislike heroics and want to try something quite a bit different.
I mean, I love Call of Cthulu, but it's hard for even a Great Old One to beat sentient psychokinetic jellyfish as a PC race. That's just too frickin' cool.
