The difference is about one pound (2.03 for MB vs 3.02 for MBP). That's not a lot until you have it and whatever else you carry in a backpack and carry it around all day. Personally, it's negligible to me, but as with most things, YMMV.
I recently made this decision. The MacBook 12 is definitely lighter, but the MacBook Pro 13 is pretty darn good as well. The benefit of 4 USB access point versus 1 on the MacBook 12 were the deciding factor for me. A couple of months later I'm happy with the decision.