Demo for sure. I personally don't feel their is a large difference in build quality or performance between the two, although a kite without the pulleys on the bar is going to feel different for sure.

The one thing I can say with confidence... don't let the 12 on the side of your Cabrinha be a barometer to judge what the wind range on (most) other 12m bow kites will be. If you do, you will end up getting all out of whack. You probably already know this.
The 12m Contra Bow is definitely more like most other brands 15m or 16m in terms of low end. So much so, that at one point, Donna had two 12m kites: The 12 Contra for 10 to 18 and and the 12m Flexi Ion for 15 to 25.

I actually think the 12m Contra was bigger because we repeatedly laid it next to other 15m kites on the beach and they were about the same size. Even if not, it sure flew like a bigger kite. Keep this in mind when doing a demo.