These match-ups are getting tougher and tougher.
Ratchet & Clank is one of my favorite franchises ever. I own every single game in the series and while they're all great, Up Your Arsenal is the masterpiece of the series. This game took the weapon-based platforming of the previous games and perfected it. The weapons great and had a large variety and the levels were bigger than ever. It's the funniest game in the series and introduced on of my favorite video game villains ever in Dr. Nefarious. Plus it's the first R&C to have a full (and insanely fun) online mode and the only that let you drive vehicles such as dune buggies. Like all the R&C games, this was just pure fun and I'll pop this bad boy in any time.
Assassin's Creed II is fantastic. It improved upon the original AC in every possible way. Better graphics, much more variety in missions, a larger weapon arsenal and a much more engaging protagonist. I've always had a love for renaissance art and history, so being able to experience that in game was a dream come true. The world was beautifully realized, from the architecture The plot was great, with The Truth puzzles and the mystery surrounding it being a highlight of mine. This game also holds the unique distinction of being the only game I have a Platinum trophy for. For someone who's never been super into trophies/achievements, that's says a lot.
Both of these games are phenomenal and frankly, I'm okay with either game advancing. But I'll vote for Ratchet & Clank here simply for because I have more of an emotional attachment to that game and the series in general than I do Assassin's Creed II. Plus I doubt Ezio's power of seduction would work Courtney Gears.