I have the Soma Mixte frame with 700c wheels and I am sometimes regretting not getting the 650b wheels.
It's not a big deal if you don't want to put on fenders, but as I need fenders The widest tyres that I can put on at the moment are 28c. It's hard to get tires so skinny suited for winter riding.
I also get quite a lot of toe-overlap which sometimes causes me trouble (but this might be because I'm just not used to it from other bikes).

On the other hand you have to consider that there aren't that many options for 650b tyres either. But this might change if they introduce the new 650b MTBs in the coming years.