How to handle getting an offer to come back?

I'm a rising junior with a couple weeks to go in my internship at a top Canadian bank in Toronto. I'm reasonably sure I'm going to get an offer to come back -- at my mid-term performance review, I was told that people on the desk were "glowing" about me and literally had nothing bad to say. furthermore, they've told me that I'm much better than the last intern they had, who they are bringing back for a co-op or FT position. The group head has been a lot nicer to me these days, and a couple associates/VPs asked me if I would come back if they offered me.

However, being a rising junior, I want to try something else next summer (I love the environment, but would like to try either trading or M&A -- right now, I'm in DCM which is more of a cross). My thinking is that I work on the floor right now, so although I love trading, it'd be worth it for me to try M&A/Corp. Fin because it would expose me to a totally different environment. And if I didn't really like that, I could try going back to DCM or S&T for FT.

I posted about this a couple months ago (and didn't get much help), but how should I go about trying to switch groups? Do I really screw myself over with my current group (i.e., they'll take it as a "Fuck You" and not want me back)?

The other thing is that I was lucky enough to get this internship, and am worried that if I get the run around and sent through to HR for an M&A position, I might not get an interview because of my shitty GPA (3.1). So in the end, I don't know if I should just come back to my group because it's the safest thing.