Personally, I feel it's better to base your decisions not on your gut feelings, but on the practical aspects of it.
Sure - you might like chemistry now, but are you sure you will like chemistry 18 months down the line? AS/GCSE Chemistry is straightforward enough to hit A grades, but A2 in EdExcel involves Transition Metals and Nitrogen/Benzene - where dropping 20 marks is 100UMS (that's how hard everyone finds it).
The best way to check is to well, have a go. It'll save you from a world of hurt if you can decisively rule out subjects that are simply far too much.
NB - quantum theory is only easy at a superficial level, it gets much harder later on.