there is a lot of skepticism surround blops. and for good reasons. the studio is officially closed once they get this game out, which reflects their weak performance IMO.
that said, noones seen the final product and plenty of us are holding out it's at least better than resistance...
also they are very very different games. do u want a shooter or a third person hack-n-slash with rpg elements? both have strong online components too.
What are your preferences, do you like RPG's more or Shooter games? If I were you, I would wait until the end of next week for reviews for Call of Duty. Do you have any interest in AC3L or Need for Speed Most Wanted? They are good games that I know are purchases.
Best thing is just to wait a couple days and pick then, after you see more of CoD and a more open view of the game once it is really released. For what it is on the Vita it should be fun, I am not looking at it any other way. I am mainly focusing on the multi-player games for the Vita as of now.
I can say I love Ragnarok Odyssey right now, so it's really what you will prefer in the end. I am more of an RPG person now a days, I used to love FPS but these newer ones aren't appealing to me but CoD: Declassified will be the best shooter on the Vita for now so that is good enough for me.
I am waiting to choose between CoD or NFS:MW.