The Mk7 is the standard model with the drift adjustable low drag rear sight and plain front sight.
The Dynamic is the same base gun with the fiber optic front sight and dynamic drift adjustable rear sight, plus the bigger mag button.
The K100 Target has the fiber optic front sight and an Elliason rear sight, adjustable for windage and elevation.
The base gun is the same. It's just the sights and accessories that change.
There is also a price leader version of the Mk7. Instead of coming with the standard backstrap and the three additional sizes, it's just got the one.
If you know what sights and accessories you want, buying the version that comes with them as standard usually costs less. Aside from the Target model rear sight, you can always upgrade a Mk7 basic with the other sights, backstraps, and mag releases later. It will just cost a bit more.
The X-Calibur has a longer slide, longer barrel, and a hammer system with redesigned geometry for an even lighter single action weight. The new hammer system is backwards compatible. With the parts in hand I can R&R the gun over or back to the standard K100 system in 10 minutes.