Hi Shadowfox,
The F-3X is slightly smaller. Interior dimensions = 8.5 x 5.5 x 9.0 inches
It is not deep enough for a SLR kit (That's also my complaint about the F). The F-4AF might be a better choice. Interior Dimensions = 9.9 x 8.0 x 11.0 inches
Also, having one metal clasp is faster and more convenient than two.
Yes, the canvas bags will mold to your body, but they also absorb water, and the black and blue ones don't age well, and look ugly when faded. The Olive Drab is a classic Leica M choice. The Sand one has a different color for the strap, which makes it look a bit odd.
On another note, the F-803 vs. J-803, the J-803 is slightly larger
J-803: 13.5 x 4.5 x 10 inches
F-803: 13 x 4 x 9 inches
That extra half an inch does make a difference to a Leica M kit.
I can't believe I know so much about Domke bags. To me a good bag is cheap insurance on an expensive camera. No need to skimp on a few dollars when one bang could cause an expensive repair.