Personally, I find biology more enjoyable to learn (apart from the rubbish how science works bits) but chemistry more challenging (which I also enjoy). Chemistry is more difficult as a subject but if you think you could do either then I would do biology.
What are the other subjects you're thinking of taking, and what are you thinking of doing at university?
Chemistry would probably keep your options open more but it is a very hard A-Level, although Biology is quite hard as well. I do both and I enjoy Biology more. If you're more interested in Biology I'd take Biology because you'll get a lot more out of studying it than you would from Chemistry.
Chemistry I enjoyed far more
Biology, if you can repeat everything you read, yuo will be good at it
Chemistry is more harder, but much better!
Chemistry would be better for showing science understanding etc; it's definitely harder than biology and holds a slightly better reputation in my opinion. If you are predicted A* in both and find chemsitry easier to do well in exams in then i would definitely go for chemistry. Biology is interesting but i don't think it has as good a reputation as chemistry and it is definitely easier.
Probably do what you enjoy if you're not strictly wishing to go down a science route.
Chemistry is more challenging and rounded as an A-level, so I would say it is more respected in general than Biology. Biology basically requires you to memorise alot of information in preparation for the exams, with very little systematic learning. The concepts in Chemistry are considered to be more difficult, and will require you to use some basic maths. In my experience, Chemistry practicals are considerably more sophisticated than Biology practicals, and consume more of the timetable. I think the course content of both are very interesting, although inorganic Chemistry is very tedious.