I did OCR GCSE and am currently in my second year of AQA Biology and Chemistry A Level and I can tell you there's not much of a jump if you change from one exam board to another! All that happened was when we'd cover one thing in a lesson I didn't know my friend would say they did at GCSE and sometimes there were things I had done and they hadn't. You should be taught everything you need to know regardless of whether it was covered at GCSE or not.
As for which is better, look at the specifications for both courses, past papers, grade boundaries and so on to make an informed decision. Personally I hate AQA just cause they're so picky in their mark schemes at times but I can't tell you whether or not OCR is like that so take my opinion with a (large) pinch of salt.
I'm currently doing my first year in AQA Bio, and although it's early to say, it is quite tricky, i'm failing just a bit :/. Also beware the 25 mark essay!