What other subjects are you doing because I'd recommend doing both as they really go hand in hand with each other.
I only do Bio and my major regret is not doing Chem because I changed my mind about doing a law degree to wanting to do Biomed and my bio teacher said I might struggle now.
Bio isn't that much of a jump imo but I have found it all easy and got an A in AS so I'd say do bio but really you should do both