Personlly, the lasts of G&G (and also EG) works extremely well for me whereas Carmina's last (heel to ball) doesn't fit as well. I also prefer the stiffer crust leather G&G uses; I find Carmina's leather to be too 'supple' for me. The soles of G&G are made from Baker (please correct me if I'm wrong) and my experience is that it lasts MUCH longer than my Carminas. Lastly, the overall look of G&G is just more angular/attractive to me. So fit, longevity, and looks are why I buy G&G instead of Carmina.
Others may have other thoughts. There are many who would rather pick up 2 pairs of Carminas instead of 1 pair of G&G.
Yes, i think a big reason why i love GG is that their lasts are simply stunning, regardless of width or size. I do like the leather quality more than the one pair of Carminas i had, and the Barker soles are incredibly durable. You can probably also add the finishing and burnishing work that is done to shoes of that price range as a plus.
Plenty of objective points of course, and it has surely been debated that the price difference in the better materials does not justify the 100% price difference, but as a visual person i put a decent amount of value on aesthetics alone. I have to, I work in the art field after all, if i can't put value in aesthetics then how can i expect others to do the same in what i do...