Ok, I think things were a little bit tougher when sussex said that to you, so in the long term the best thing for you in my opinion is to choose leeds, as tbh sussex haven't guaranteed that you will get into the 2nd year of medicine***, you should try and think what you want to do in the long term.
I think tho that you really need to decide what you degree you want to do based on what you like better, as you could do a degree, that's good for medicine and not get a 2:1 because you didn't like it.
*** BUT if they can truly guarantee it, then go for it! Do just a little bit more research on it, find out how it works would you still have to do an interview, if you are up against others in different courses etc...
Hope everything goes well, and good luck in the exams!