Only major difference is that the 9600 XT has a core frequency of 500 MHz (as opposed to 400 on the Pro), but memory frequency remains the same.
The performance difference in games between the Pro and XT is anywhere from 2-5 fps, depending on the game, though games that are very dependent on raw fill rate (e.g. Q3A) will change the difference to as high as 20, but seeing as both cards run that game over 100 fps at high resolutions, the actual noticeable difference is only negligible.
If you need all the performance you can get off a mid-range card, and $40 doesn't seem like a dent to the wallet, then go for the XT, but I'd for the most part pick the Pro.