I managed to do both assessments before finally deciding on OAT. Obviously, the first thing to say is that both assessments do have demanding elements. However, I would believe that OAT is more challenging to get into.
For Cranfield - the assessment takes about half a day and consists of the following:
Maths paper - 90 minutes - 30 questions. (sounds easy, but believe me, you do end up using the full 90 minutes)
Physics paper - 90 minutes - 30 questions
Pilapt testing (another 90 minutes or so!)
And a debrief/interview. Although this was not thorough, it did go over your results objectively.
For OAT, the assessment is on a much larger scale and takes two days. It consists of the following:
Compass Aptitude testing (90 minutes or so!)
Simulator ride on a Seneca-type IR Sim (Probably about 25 minutes, but flies by)
Team Exercises (two situations where you have to demonstrate the ability to work in a team)
Group discussions (two themes or so - 1 aviation related, one current affairs)
Interview (about 45 minutes)
and then finally a debrief where the decision is made.
I would say that in terms of assessing you, the OAT method is much more thorough than Cabair, as they examine several more key aspects.
Out of my groups from the assessment, everyone got into Cabair, and only 50% got into OAT. The other 50% were invited back in 6 months time for reassessment (although I later learnt that one was not at the standard at all).
So that's about it.
Cheers,
CJ