The first thing to do (if you have not already) is to check with local governments and find out what local codes require. Every area is different and it ranges from "any backflow preventer" to "Part Number 1234 made by XYZ company".
Other wise, the layout of your system might dictate which is needed. If you have any land that is up hill from were you plan on putting the BF, then PVB can't be used because it must be higher than the highest sprinkler head. If either one is still fine, than the PVB is cheaper. The other thing I can think to try to consider is winterizing. A double check can be burried or installed in a basement such that it's protected from the cold, but a PVB can't be enclosed.
