I've had both.
For me, the stand-out differences were: overall design, screen size and keyboard.
The MB12 is a thing of beauty. Just a stunning piece of design. It's lightweight, too. I personally love the new-style keyboards: they really increase my typing speed. BUT the screen is only 12-inches.
The MBP13 is great, reliable, full of ports. But -- in my view -- not as sexy. It's heavier and has the old-style keyboard, which is fine, but I don't like it as much as the new.
For what I use computers for, processing speed is irrelevant. Computers are so fast nowadays that I just don't notice any kind of processing delay. But I'm not a gamer.