Tom's Hardware have just put up a 21-page P35-DDR2 eight mobo megatest on their site. The performance difference between all of them would be hard to call. Advice to P5B owners is not to upgrade but to wait for the "Eaglelake" boards coming next year and then jump to DDR3 which will be much cheaper by then and will have tighter timings.
I'll be investing in a P5K Deluxe next month for my first ever build. Not because it is any better than the other P35 mobos out there but because here in these forums we have access to the guru-like knowledge of Clunk, Supershanks and others who are always prepared to do a bit of n00b hand-holding. In a Churchillian voice I say "Never have so many owed so much to so few".