Do you have an Apple Store nearby? Or anywhere that sells Macs? You should definitely go and play with both of them. They should let you close them and lift them and see how noticeable the difference is. Personally, after having used a 13" white MB and then MBP, both of which weighed the same as this current 15" rPro, it's not hard to get used to for me. I only had Airs for about 2-3 years total before I got this so I guess I never got too used to the super lightness. But I will say, the Air's lightness is amazingly awesome. So don't play with those, you might regret going Pro. lol
Curious, is there a reason you wouldn't consider an Air? They really are super awesome machines. And even if they're not the powerhouses the Pros are, they're still super fast and super light. So light that you feel like you're just carrying a few pieces of paper everywhere. I assume you use some Pro apps? That would make sense. In which case, either Pro will work fine for you. Just play with them if you can and see if you can tolerate the 15". And as you said, you can always return it if you do go 15" and decide it's too much. If you can, talk to the clerk and get his input. You're going to buy one of them either way. You just want to make sure it's the one you're going to love. And they'll probably be glad to assist you in the decision.