Ideally find a shop that carries all the PFDs that interest you. When I bought my most recent PFD there was a significant difference in the way they fit my body. Once I figured out which ones worked on my mature body, I was able to choose the one that had the features I wanted in addition to proper fit.
Take the clothes you plan on using for paddling in both warm and cold weather and make sure your new PFD fits properly at the extremes.
Sea Kayaker Magazine