I have the F4 with kydex also, and in my opinion, the F4's sheath is better. Both sheaths have good retention and good ergonomics with thumb ramps.
The F4 has a fold-over sheath with better fit. The F4 fits dead-snug, no rattle, easy draw, and it has a groove along the spine edge that makes it easy to locate the knife back into the sheath without looking.
The F3 sheath is a 2-piece sheath. The one I received has a lot of rattle between the knife and the sheath, and it's a bit more difficult to guide the blade into place. I tend to try to avoid the loud pop of inserting the knife by easing it in past the retention point while applying backwards pressure with my hand wrapped around it, and I often end up pinching the skin on the side of my pointer finger between the sheath and the knife's handle. This isn't an issue when I have a belt clip securing the sheath in my pocket or on my belt, only when I'm trying to carry without a clip in my pocket and sheathe it with one hand.
I was able to fit an UltiClip Slim 3.3 into 2 holes on the F3 sheath in both near-vertical and canted angles.
All-in-all, I call it a good sheath with nicely placed holes, would bump it to 4 stars if it didn't rattle, 5 for a fold-over like the F4 has.
