For gundam, both games are very different, do you prefer arcade fighting game or grinding/customizaiton semi monhun game? I own Gundam Breaker, I really enjoyed it, very import friendly, high customization level, really challenging (on later stage), grinding for rare parts/weapons. It a dream game for gundam/gunpla fan =). Gundam breaker is a good game but with lot of flaws, the level of drop can be really frustrating, the list of gundams or enemies mecha is not a lot, etc. On the other hand Gundam vs extreme is a lot more polish on many levels, along with bigger roster. Though I don't own a ps3, I have been a fan of Gundam vs. serie, ever since I owned Next plus on my psp. I would recommend you to get both, but if not, then I personally would choose gundam vs extreme.
hopes that help (sorry for my bad eng)