A Crack in Time, imho.
While I haven't played Assassins Creed 2, I have played number 1. I found it incredibly repetitive and I didn't finish it.
Ratchet and Clank on the other hand, great story, gameplay, humour, weapons, optionals and replayability. Not to mention skill points and trophies for extra collectables. And in this one, a reason not to do challenge mode the first time you beat the game, and instead wait until later. There's more to do good sir!
That being said, you're getting the other game in a few weeks. Since you love both games, BUT want one that'll last until you can get the other, Ratchet and Clank seems the better choice. Both in the actual game quality in my opinion and length.
1 day / Sydney, New South Wales