If I was buying a board with the possibility of CrossFire at some point in time I wouldn't be limiting myself to x4 on one of the slots when x16 was an existing option.

The two are pretty even as far as overclocking potential. If I had to give the edge to one it would be the P35 as it seems slightly better with the memory side of things. The X38 was developed for DDR3 so naturally there was some compromising that needed to be made with DDR2 performance. The plus side of the X38 is the straps. There's very little performance difference between strap changes. I tend to look at the different settings as just memory dividers.