Old thread I know, but I just finished R&C: Up Your Arsenal yesterday, and today started to play Jak 3.
Both games are awesome, people won´t regret to buy any of them. I always liked the R&C series better, but I must say I´m very impressed with Jak 3. It has one of the most beautiful graphics on the PS2, if not the most. Ratchet UYA has great graphics too, but it´s not quite on Jak´s level. But then again, Ratchet & Clank runs at 60fps while Jak3 apparently runs at 30.
I guess I was so surprised with Jak3 because Jak2 didn´t caught my attention. I started playing the game, but never got to finish it. I started playing Jak3 today just to get a grip of the game, you know, only for a few minutes. But I ended up spending the last 5 hours on it, and I just stopped to eat. It´s so damn fun!
Anyway, they´re both very good and each one has its advantages, so I don´t know which is better. Ratchet 3 runs at 60fps, it has a lot of different and cool weapons, it´s more shooter based action, it has a great online mode, and it´s bugless.
Jak 3 has slightly better graphics, a huge city, no loadings, its missions are more varied, it has some cool weapons too although not as much as Ratchet has, but sometimes there´s some graphic bugs.