I think this really depends on what else you're going to be using the computer for. If you're going to be doing a lot of serious video/photo editing then you might consider getting a Macbook Pro for its enhanced graphics capabilities. Also, if you think a 13.3 inch screen might be too much of a strain on your eyes, you might benefit from the bigger 15" screen on the Macbook Pro. The ATI graphics chip also helps gaming performance if you're interested in that as well. There are a few other benefits that I think you'll find if you just Google.
However, if you're not doing all that much video/photo editing and just need the computer to do things like store photos, write papers, accomplish "normal" day-to-day activities, then you might be better served by the Macbook. I think a top of the line Macbook will more than suit anyone's needs for BackTrack 2. I'm using an almost 3 year old Sony VAIO S270P (1.7ghz Centrino, 1024MB RAM, 80GB HD, ATI Mobility Radeon 9700) and BackTrack 2 runs without a hiccup on it. I'm also part of a research team at Carnegie Mellon University that is doing some penetration testing research in the local neighborhood and I haven't had any performance problems at all.
My suggestion is that you read some reviews on the Macbook / Macbook Pro and see which one better suits your needs. I myself am anxiously awaiting for the next generation of Macbooks to be released. Good luck buying your computer!