The difference is levels of refinement of design.
first came the S6 in 2002
then the S6X in 2003
then last the S6F in 2004
With the original S6, you have a nice little boat in a couple of different sizes (190 and 200), nice and narrow so it carves quickly from edge to edge when surfing. It's got some breaker lines across the hull so that there is kind of a spin disk sort of thing under you, which makes it pretty loose on a wave. Pyranha also added loop pockets to the front and rear, they look a little like gills.
But there weren't enough sizes for everyone, so along comes the S6X. You get the 185, 195 and 205. Pyranha took away the breaker lines in the hull (made it smooth tip to stern) added a bunch of bolt-on doodads, like a half dozen fins and a mystery hull plate and some loop pocket enhancers. They also upped the rocker, kicking up the nose and tail substantially. A good boat, it taught me how to loop.
And now along comes the S6F. All of the doodads from the S6X are gone except for a pair of fin mount points. The hull breaker bars are back, and the rocker has been mellowed down a smidge. There are still 3 sizes, and they are 191, 192 and 193. It looks like a solid design to me.
All the variants are pretty quick edge-to-edge on a wave, and in terms of looseness, I'd put the F above the original S6 and both above the X. As always though, comfort and boat fit comes far above boat design. Getting the right size boat and adjusting it to fit you properly is crucial.