You are right, kydex sheath is more wear resistance and probably will last longer (how longer?). It also is easier to clean, it doesn't afraid from water. On the other hand it can become somewhat fragile (again - how much fragile? - certainly not like glass but more fragile than leather one) at the low temperatures. Another disadvantage is that it is much louder rattling against the gun or another hard parts of equipment (how important it is for you?). Each sheath will scratch the blade if sand grains will get inside, kydex one does it more than leather.
Next - both kydex sheath and leather one with flap position the knife with the handle's butt on belt level. The open-top leather sheath dangles noticeably lower. This allow you to get the knife easier being clothed with heavy winter jacket but the knife will swing and hit your hip when running.
Next - the kydex sheath holds the knife as itself enough securely to do not fasten the safety strap if you are not parachuting, jumping into the water or do similar things.
Here is not direct advice, just think what is more important for you.
How long will last black teflon coating? Hmm, it will last... Another thing is how it will look being scratched in knife use... If normal knife use (skinning, gutting, food preparing, wood carving, etc.) it probably will look not too tragically. Prying, pounding, digging and so on it could look much worse. Again - think how important it is for you.
However VG-10 steel is enough rust resistant, so if you will take even minimal care to your knife and if it will not frequently contact with salt water - it shouldn't cause you too much problems even with uncoated blade..
