I personally didn't find AS that difficult - I was prepared for something impossible, considering the number of people who told me it was difficult and the number of people at my school who failed. But it's OK if you put the work in. Some of the concepts take a while to get your head around, so you'd have to work to get those into your head (people who never get their head around will find Chemistry seemingly impossible). There's also a fair amount of memorisation to do, like industrial processes, organic reactions, trends and patterns, etc.
The good thing with Chemistry is that once you understand the concepts, there isn't as much rote learning - you can apply the same kind of knowledge to lots of things (e.g. the concept of charge density of an atom can be applied to their solubility, reactivity, thermal stability, strength of bonds etc). With Biology, on the other hand, there is a huge amount of rote-learning to do which just has to be learnt, and you cannot get away with applied knowledge from elsewhere. In that respect I found Chemistry easier; also because the questions are more predictable than Biology, and you can usually guess the points that the marks are allocated to, unlike the randomness and utter shambles that is a Biology mark scheme!