On the outside you may not notice a huge difference in these cars... but the GTR is in a totally different class. First of all the GTR has a V8 in it... and were not talking the V8 that goes in the M5... this is a totally different engine. It has dry sump oil lubrication for one, something you will only find in the best race cars. Surprsingly, the CSL actually has more horsepower putting out a whomping 360 whp from just the I6 (that's a BMW Motor Sport Engine for yah... amazing!). Whereas the GTR puts out about 350 whp from that sweet V8... but, don't let this decieve you. This car can haul, and it only puts out 350 whp because BMW had to put major emissions on that car to make it streetable. I am sure that with a crew of guys who know what they are doing, 400+ whp can be reached from that MV8. The race version puts out about 444whp, but can honk 0-60 in just over 3 seconds. that is faster than most Ferarri's and Lambo's that are putting out 550+ whp. that is actaully the same as the Enzo... the million dollar car... hold onto your seat though, because their is a lot more that is different between these two cars, which are both sick by the way, and if you could get your hands on one, you would never go back.
Moving on, the CSL is basically a stock M3 that has been gutted of all luxury parts to make it substantially lighter. Then it also has some tweaking done to the stock M3 I6 that puts out around 333whp, to get it up to 360whp. The CSL, which I am sure you know has a Carbon Fiber roof, special glass-poly fibre composite seats, and even optional CD player for further weight reduction... ah heck, I could write pages about teh differences... if you really want to see the differences read the following two web sites and compare:
For the CSL:
http://www.bmwworld.com/models/m3_csl.htm
For the GTR:
http://www.bmwworld.com/weeklypics/20011020.htm
or
http://www.rsportscars.com/eng/cars/bmw_m3gtr.asp
These are both amazing cars, and I think they would be very tight in a race environment. The GTR is my dream car simply because it is rocking the Le Mans scene everywhere, but the CSL is really the true definition of a sports driving expereince for a street car... I could say a lot more, because these cars are truly mind blowing, but I don't want to bore you with my obsessions for these well built machines.
Hope this helps...
Josh