The main things to look at are the heaviness of construction and accuracy of fit. A flimsy fly cutter is going to be hard to get good results from - period. As for fit, in some fly cutters, for instance, the way for the cutter blade is oversized. If the setup for tightening is above average, this may not be a problem. But it is generally better to have the cutter blade fit quite snugly into its way even when the set screws are loose.
Cutter blades in most fly cutters come with a quite sharp pointed end.
This is great for wood work, but not so great if you are cutting shoulders in PVC, because the shoulder will end up needing to be squared off by hand. For doing the kind of work we are specializing in here, the end of the cutter blade should be square when viewed from the front - the front being the side that strikes into the material first. The clearance, as seen from the side, should be in the range of 5 to 10 degrees. This will make it much easier to keep the material solid in the holder. Experts might correct me on that, but this is MUCH better than the 50 to 70 degrees some fly cutters come with.