I was fortunate to acquire an FPJ CB prior to the piece becoming extremely difficult to source.
In terms of how I decided on the specific FPJ, I feel this is specific to each individual in terms of what type of timepiece, complication, metal, etc. he/she is looking for. In my specific instance, when I started looking at FPJ I had acquired a number of other complications from other manufacturers and was looking for a time-only watch that could be used for both semi-formal occasions (business casual work) and leisure (shorts or jeans with t-shirt).
With regards to size, I would highly recommend that you try on the timepiece prior to purchase. Some watches will be 40 mm but look bigger while you may also have a piece that is 42 mm that appears smaller on your wrist.
Having a relationship with the Boutique would be helpful in the future for limited edition pieces. I have also purchased watches from my local AD (not FPJ) that helped secure other watches that were more difficult to source. In the case of my FPJ CB, I purchased directly from the Boutique and the experience (pre/post sales) was very smooth and pleasant.