I purchased both, because as newbie008 said, you only get one chance.
Pros and Cons
Pilapt Prep - has many of the tests that CTC use. The only downside is that it does not have a deviation indicator test or the trax (flying through boxes test.) The other downside with Pilapt prep is that it doesn't score you so it's difficult to judge your own performance.
Cockpitweb is very good however the hands game is much easier than Pilapt prep and CTC's, and it doesn't have the grid task, and the grid task (called concentration on pilapt) is very difficult. The other downside with cockpitweb was that I couldn't get the flying through boxes to work.
Unless money is no object I would recommend getting pilap-prep and using Microsoft flight sim to practice the joystick skills.
Be warned however the deviation indicator is extremely difficult to keep centred and at times I was at full joystick deflection and still I couldn't centre it.