Things to consider are -
(1) whether you would like to take on the industrious ethos of Oxford, where contact hours with academics are high, workload is heavy, and your peers will be people who were at the top of their school's ability range;
and
(2) whether you like the idea of a college, which is a small self governing academic community and not, as some people who may not have been undergraduates at Oxford or Cambridge may suggest, a hall of residence with extras.
Chemistry students at Oxford have a busy schedule, and students of every subject are expected to perform to high standards, but Oxford undergraduates also have time to make friends and have fun.
Manchester is a good university in an interesting big City. Oxford is Oxford. Visit both and try to get a feel of which you might enjoy most.
Please don't let parents or anyone else choose for you. Do what seems best to you.
Good luck, whatever path you take.