I use both, but it depends how I'm using the camera to determine which finder I use.
Mamiya 645 with power winder (or in portrait orientation): prism
Mamiya 645 w/o power winder (landscape orientation only): WLF
Bronica SQ-A w/ Speed Grip S: Prism
Bronica SQ-A w/ handcrank: WLF
Bronica SQ-Am (builtin power winder only): Prism
Mamiya RZ67: WLF mainly. This is because the AE Prism weights as much as the body, 110mm lens, and a back by itself. I did use the Prism when shooting high on a tripod.
So pretty much, if I'm using the camera with my hand orientated like I would with an SLR, I'll use the prism. Or if just using the front shutter release and hand winding, I prefer the WLF.