I do both chemistry and physics, and they are just as difficult as each other in my opinion, maybe physics a little easier. Don't take your GCSE grades to be indicators of how well you will do in each as many people get really different grades to those at A level. There isn't too much between an A and B anyway and it's not like you got A* amd C. Chemistry is more of a fun subject, I think the practicals covered are more interesting but that's obviously just an opinion. Just do what you think you will enjoy most☺️