But there was nothing to change though, that's the problem. The keyboard has been updated to cater for new buttons (as in, updated keycaps), the trackpad had nothing to update as Apple haven't made any changes for the last 6 years or so (Force Touch is the only thing they can really do, if you think about it), and the mouse is always going to be awful anywayThat reminds me to have a moan at no-one in particular; as an owner of the mouse, keyboard and trackpad it's a bit annoying they haven't updated the peripherals in forever.

I'd agree with this really, though whether you look back depends on your priorities. Personally still prefer the Air's keyboard and taper to the rMBP, I found the rMBP to be closer to the cMBP's form factor in that your wrists are very high when you type. Didn't like it on my cMBP 13", and didn't find it much better on the rMBP.You will notice the difference, not just the weight but the form factor as well. I really liked the way the Air tapers, but ultimately after a couple of weeks of usage you won't look back I don't think.
In terms of using the machine, my opinion is that if you primarily type, the Air is possibly the better choice, purely because the keyboard has a little more travel, and has that taper (which makes typing effortless). If you primarily browse or edit pictures and videos, probably the rMBP.
In terms of the actual form factor and carrying the machine, in a bag the MBA will probably be noticeably smaller but not really feel that much lighter. Carry it in your hand (in a sleeve), I'd take the Air.