Crossfire performance on 975X is much better than it on most P35 boards, because most P35 boards run one of the video cards in x4 mode, while 975X is x8/x8. X38 will be a full x16/x16, which may be a shade faster even than 975X.
X38 also offers superior over-clocking capabilities to P35, some tricks with DDR3 memory that P35 can't do, and some additional optimisations for Penryn processors (allegedly).
Bascially X38 is for the "enthusiast" sector, where P35 is for the "performance" sector. P35 is for people who buy fast cars; X38 is for people who buy fast cars and then fit them with nitrous oxide rigs. Most people can't be bothered doing that. X38 is for those who can be bothered.