Once you are a doctor, no one cares about your medical school grades. Cardiology is competitive because often people will have done a PhD before applying and that would confer them more points. Other specialities like psychiatry, GU Medicine are less competitive because they are either not well known or people dislike some aspects.Things are changing anyway, for example now all those doing a medical speciality will also be trained in acute medicine. That means more on calls and the medical registrar job is well known to be the hardest.For surgery you will need to be a lot more academic but if you love the speciality you will just get into it.I would suggest first going through medical school and then see what topics you liked.