I'm a pre-flasher...
Everything I've read (until today) has said that pre and post flashing have the same effect, although I've always thought it must have some however slight differences. never seen them myself as on occasion I have completed my enlarger exposure and think, oops, forgot the pre-flash so I post-flash and stil get the print I was aiming for. But if Les can demonstrate a difference, I'll believe 'ya.
If you are going to flash the paper, you must test to work out your enlarger/paper combination, so if you use two papers, you need to do two tests to determine the appropiate time for each.