The differences aren't massive, they're more to do with layout than anything. The difference between CRP-1 and CRP-5 is the speeds you can calculate up to are much higher (i.e. for jets) and a couple of extra functions for commercial level.
I would try and pick the same computer as the instructor (if they use one) so then they can explain how to use it much easier than having to familiarise themselves with it first.