If you want retention, go with the Serpa. I've seen firsthand a Serpa lock up due to a little dirt/sand preventing the pistol from being drawn in a timely manner. While I was on active duty, my unit used to issue Serpas till they started getting dirty and locking up...sometimes having to take both hands or even taking the holster (with gun still intact) off and washing the dirt/sand out. The company I work now for won't allow the Serpa for this reason as well.
"Four wheels move the body. Two wheels move the soul."