FWIW, the guys over at Bare Knuckle Pickups seem to generally recommend using an F-Spaced bridge humbucker (aka Trembucker) when replacing stock pickups on a trem equipped Custom 24. On the same topic, if someone were to replace their bridge pickup (85/15) on a Custom 24, does anyone know if an F-spaced humbucker will fit in the standard PRS pickup ring designed to work with the current generation of pickups (85/15) with the squared end bobbin covers?
From what I remember reading, uncovered f-space will fit, but there may be an issue with a covered f-space pickup. Again, there's no need to use an f-spaced pickup on a PRS guitar except maybe if it has a Floyd.
Measure the string spacing over the set of pole pieces closest to the bridge. That will help guide you on how well the pole pieces will line up and will help inform your decision on which spacing you want.
The current PRS made rings are designed to fit a standard covered humbucker. The uncovered "squabbin" pickups are designed to be the same size and shape as a covered humbucker so it fills the inside of the ring and looks "right".
This looks helpful: http://forums.prsguitars.com/threads/trembucker-pickup-rings.21561/#post-314201