I've not played Jak X yet, so I can't comment on it.
However, after playing GT4 for a few hours, I can certainly say that it's essentially GT3 with a bunch more cars. That being said, as realistic as people try to play it of as being, it certainly is a far cry from realism in many respects, and can get quite boring and tedious when you start running into the less fun unrealisms (such as the joke that the game passes off as "rally" or the 5 second penalties the game gives you for hitting invisible barriers or when the computer oponent hits you).
I could go on, but I'll wait until I can collect my thoughts a little better to write it out.
My conclusion? Buy GT3 and experience, essentially, the same game, and buy Jak X for something on the other side of the spectrum (or get Burnout 3, which kicks copious amounts of ass).