I was in the same situation a few weeks ago. I had a choice of going to one college or going to another, in which the Physics exam board was OCR A and AQA A. I chose the other college as it is closer and more convenient.
I did OCR for my 3 sciences at GCSE, and I believe that it is the best exam board for any science at both GCSE and A Level. OCR A has a paper on mechanics solely at AS, whilst on AQA A it is incorporated into Unit 2. If you are confident with mechanics, then go ahead with OCR. OCR also tend to have lower grade boundaries compared to other boards at A Level, which is a bonus as it all really comes down to how much work you put in. As for the second unit, OCR and AQA A have identically the nearly the same content ( theres a few differences as you would expect ) but it depends on the way the exam questions are structured as that is major difference.
I think that you should have a look on the specifications of each course and look at the past papers and compare to see which suits you best content wise and how you can handle the exam technique. If I was you, I'd go with OCR A, or the exam board that you was on for GCSE. I think OCR A are the most cogent and have many resources.