A hard question to answer because most of us will have only used 1 exam board. but i guess you can compare our expierences.
Im on AQA atm and the modules look like this:
AS:
Unit 1: Foundation Chemistry(100 ums)
Unit 2: Chemistry in Action (140 ums)
Unit 3: ISA(practical which involved a written examination a few days later, 60Ums)
A2:
Unit 4:Kinetics, Equilibria and Organic Chemistry(120 Ums)
Unit 5: Energetics, Redox and Inorganic Chemistry(120 ums)
Unit 6: ISA(60ums)
the ISA's are hard and im not sure how you will be able to do them if you are an independent candidate. the Exams can also be a challenge as they rely less on deffintions and more on application(nowadays anyway). Now i dont know alot about OCr but i do know when my chemistry teacher was told he had to teach for the OCR exam board he said he wouldnt do it and they would have to find another chemistry teacher !!( bit extreme i know) his reasoning was that the OCR exams were very poor in spreading out the content in the specification in the exams, for example they would dedicate 1/4 of the exam to 1 particular topic when there were 8 topics to pick from. His only critisism of AQA is the ISA and hes not the only 1 who thinks AQA ISA's are horrible. In the End you should probly pick a coursework based exam board that doesnt rely on practicals, maybe edexcel does CW?
Edit: Just relised you are planning to do the whole A-level in a Year :O? it might be possible but i dont think you will be able to achieve an A imo as A2 is a complete beast and without time to chill out you are gunna be overloaded by work.