They're just about equal now. P965 is newer among the two and probably overclock better. 975X is somewhat more versatile (native PATA and 8x8 PCI-E). Other than the slight feature sets, you can't go wrong with either chipset. What's rather important is how individual boards implemented the chipsets. Check the motherboard reviews and you will easily find the trend.
P35, a succesor to P965, is about to launch and you can actually buy now, albeit with ridiculous price premium.