I haven't used the MB 12" very extensively (just in the Apple store for a while) but I'd shy away from it for several reasons. I do own a 2013 rMBP and I've owned a 13" MB Air as well.

Connectivity - Sooner or later you're going to want to plug something into it. This might be the TV in your client's conference room. It might be a HDD, your phone & the charger. One USB-C port is going to make that at the very least difficult and you're signing up for adaptor hell. Sure it's "only" $80 for an adapter but those will add up quick...

Keyboard - I can't stand the keyboard on the MB for a decent amount of typing. Some people love it, others don't so your mileage may vary BUT if you need to write code and you hate/aren't accurate with the keyboard your only real option is an external KB (see above).

Screen - 12 inches is quite small, especially with the higher resolution. I think you're just asking for eye strain when you're writing code or dealing with the Sketch interface. That diagonal inch on the rMBP makes a bigger difference than you might think. You could always plug in an external monitor... Oh wait Apple doesn't offer a USB-C monitor yet so even with an adapter you'd lose the benefits of the extra connectivity...

Honestly I think the price on the MB 12" is way too high for what you actually get (specs wise). It's not significantly more portable than an Air and the Air has more connectivity options. Plus you get quite a bit of computer for the price in the Air.

If you want portability above all else I'd look at an Air. If you're going to use it very much (which sort of seems like the point) then I'd suggest going with the rMBP.