I am no expert but I have spent some time on a hobie. I own a Hobie 14T, it is a great solo boat but... becomes somewhat of a sub after you put 2 on it. If you are going to be solo 80% of the time go with it. Make sure you get the turbo, the regular is hard to tack.
I have been on the 16 a few times. It is a great 2 man boat, but lacks in the solo department. First of all, it is hard to right alone without some kind of aid. Second of all it takes away from the simplicity of it.
As for the 15, we don't have those around here.

As a last comment I would like to say that I am trying to sell my 14T so I can upgrade to a Nacra 5.2. I have run into the dreaded disease called onefootitis, I think the nacra might cure it since I am upgrading a couple of feet. You might look into a Hobie 17!