Been as you are only at secondary school, don't know how old though, you still have time to decide for definate what you want to do. Nobody will be able to advise you which one to do, its you who will have to do whatever you decide in the end.
Having said that though!....
They are two very different areas to enter so you need to be sure which way you are heading, esp when you choose A-levels. Someone how does/did medicine will know more, but I think they want mainly sciences for that, whereas banking would want more maths skills. You will also need to get some experience in the medicine field if that is the direction you want to go. It may be worth you trying to get some work experience in each area, which may help you make your mind up more. I know some places aren't as helpful giving w/e to people still at school, but if you can compare the actual work involved with each job you will probably realise which is for you (if either is.)
You mentioned the pay for the two areas, both are fairly well paid, but pay won't matter that much if you really hate a job, but are earning loads. Also, I doubt if potential patients/clients would be impressed knowing that you don't like the job and are only in it for the money. You could qualify in several years time, not like the job and then you will have wasted time and money getting somewhere you dont want to be. If you are saying that you are leaning towards banking already, then maybe the medicine route isnt for you!?!
Like I said, you still have time to decide for definate, but give it some proper thought and don't just pick job ideas for the money. Think about if it is really someything that you can imagine yourself doing for a very big part of your life. You could even look at some of the univeristy websites, most if not all, have online prospectuses, so you can have a look at what the degrees would involve.
Basically, research all your options and then come to an informed decision!!!