I'm not sure exactly what a 100 pounds is in USD, but here are the advantages and disadvantages of the 2900 Pro, as far as I have discerned.
The 2900Pro is pretty much an underclocked version of the 2900XT, so much so that they can even be Xfire'd together. This brings with it the horrible power consumption that the 2900XT has, and with more power, comes more heat. It's a little bit slower than the 2900XT at stock, but an easy OC can bring it up to speed and above the 2900XT.
It all has to do with your case, your power supply, and if you are willing to have a really loud fan.
You should probably list your specs, that way we can help you choose between the two.