I have a 15" Macbook Pro Retina and it's perfect for what I do. Very light/portable and has a bigger screen, which I also like too. After going retina, I would never want to go back because I notice every pixel now. Not anymore difficult to design on than a 1x computer either.
Same that I use - use to have a 13" Macbook, but when upgrading, went with the 15" and I've found it's much more comfortable for daily use and work while still being small enough to just throw into my bag to carry around. Plus, the guts are a bit stronger so I can play some games and stuff on it too without worry.
I have a 15" Macbook Pro Retina and it's perfect for what I do. Very light/portable and has a bigger screen, which I also like too. After going retina, I would never want to go back because I notice every pixel now. Not anymore difficult to design on than a 1x computer either.