Two different genres. It's like comparing an Evo X to old school GTO. I haven't played either, but I'll probably have to go with FFXIII since that was the whole reason behind buying my PS3... I'm more of an RPGer anyway.
Well, considering I've got FF13 I'd have to say that you go with God Of War 3. FF13 has the worst battle system I've ever seen...It's almost like a repackaged version of FF12 :/ Seriously, don't buy it!
If I wanted to play God Of War, I'd save my money and just replay GoW:II. FF13 can be a bit dull but atleast it's trying to introduce new ideas and systems to the series.