Regardless of what you do, you should hurry up a little. F.e. GHL already closed their graduate applications and will hold its AC at the end of September. Other banks also hire on a first come first serve basis.
Maybe you could split your applications into a few graduate apps and a few summer internship apps. If you were to secure a graduate role, then obviously take the FT job over an internship opportunity. If you only score internships, then go for the firm with a high conversion rate, f.e. JPM amongst others. Btw, some companies such as JPM or RS make this decision for you, since they don't hire any or only very few people for their graduate program and instead tend to convert their interns. Hence, I would apply to both banks for internships.