i have just purchased the above games for the PS2 i have heard that both are very difficult which would you say is the better of the 2 games?

Can't really say as one is a fighting game and the other is a racing game.
In my mind they are both about the same in fun rated at 8.5 to 9 out of 10. Then again I guess that this doesn't matter if you have purchased both.

just a word of advice, if youre not into difficult games then dont go for VF4. But then again its a very deep game where you gotta learn the patterns instead of smashin buttons so i feel its better than wipeout.

VF4 is a challanging fighting game, its good though, I just finsineh playing it.