2.0....
I own the 2.0 and am extremely happy I didn't get the 2.4. The 2.0 is lightning fast and you will not notice a difference in speed unless you have the two of them next to each other and have a stopwatch. Even then you are talking 3-10 seconds depending on what you are doing.
Here is the link to benchmarks compairing both models:
Benchmarks
It comes down to this, do you want 90GB bigger hard drive and backlit keyboard for 300 extra dollars?
Yes or no?
If yes, get the 2.4.
I personally have a 500 GB external HD I use so space is of no importance to me. I would say that you should upgrade the RAM to 4 GB as I did especially if you plan on installing Windows via Boot Camp, VM Fusion, or Parallels.
This is my post in the other thread mentioned earlier if you haven't glanced at them:
"After reading all of the threads about 2.0 vs. 2.4 I can now post with confidence that the 2.0 GHz has more than enough processor speed to accomplish a myriad of tasks at one time. With 4GB of RAM I am utilizing 10-15% of CPU and 2.13 GB RAM at my maximum usage thus far (transfer of 35 GB of music onto Macbook Alum and then putting them in iTunes while running to test speed 10 other programs. I would recommend that if you are torn between the two get the 2.0 GHz with upgrade in RAM, it is plenty fast and has ample memory. You will not regret it."
Hope this helps.