Which version of each are you contemplating?
CoC has a big, thick, tome called Basic Roleplay which enables you to run a wide variety of time periods and flavors of "cthulhuoid" games, from 1920's to sci-fi, with magic and mutations and psionics all on offer. You can buy expansions to cover Roman, Dark Ages, and other eras. I like CoC but I have to say if you use the combat system fully it can get a bit intricate. Roll to hit, roll to parry, remember to check for success, special success or critical success, compare rolls, roll on crit charts if needed, track weapon and armor damage, etc... it can take some getting used to, especially for DnD players. But it is fun and can be played quickly.
Traveler has several rule versions. The most familiar to me is the old black-book set, or the first revision. Rolling up characters is loads of fun, but I always found the lack of later advancement to get a little boring. For one-shots or adventures interspersed with other games, that may not be a drawback. However, the game really ONLY supports one genre of play - science fiction. And it was always BEST for us played as "travel in space - have an adventure on a planet and leave - travel to a new planet". We never seemed to do campaigns, just episodes...