What sets these modules apart from others is their ability to run at high speeds with lower latency settings.
I've used several XMS modules and find them to be of extremely high quality and very overclockable. Currently running 256MB of PC3200C2 which does 200MHz at 2-3-3-6-1 as promised by the manufacturer's specs.
I have not tested any Geil modules but I have used Mushkin, and while I consider Mushkin to be fine memory my favorites based on personal experience are the Corsair XMS modules.