I don't think that it's the insulin as much as the food that would cause weight gain so I'm not sure that metformin would help if your BG are ok.
What has helped me with the pump is that it keeps better records than I do about what I'm eating. I also started tracking my food w/ this app called "Lose It" which made me realize that I was overbasaling a shade, running lowish a lot and munching like 15-30G of jelly beans to maintain my BG. I turned the basals down maybe 6-8 weeks ago and seem to have saved myself those calories, not a huge thing but I've been plateaued weightwise for a while and would sort of like to drop maybe 5-10 more lbs? The tracking of the pump maintained pretty decent records of what I eat and I've hit other plateaus and said "hmmm, maybe if I cut out _____ and replace it w/ something less carby, I can lose some weight" and that's worked well for me?
I'm not sure about Omnipods but having read about the issues people with them seem to have, I am sticking w/ MM because the tubing doesnt' bother me at all.