It really depends on where you're gonna be and what you're gonna do.
For instance, if you'll be in a hot humid place that gets alot of rain,and it gets wet alot, like Viet Nam, most leather will rot or fall apart fairly soon. There are some treatments you can use and maintenance that will prolong it but in this kind of scenario kydex would hold up much better.
However, if you're gonna be in any kind of situation where stealth and silence is important, kydex is awful. It makes noise removing and replacing the knife, it bangs into things, etc. It makes no sense to me that they use if for so called sentry removal knives or other sneaky tasks.
If it's just for general use in a friendly environment where you can take reasonable care of it, then I think go with whichever you like better. Theoretically of course the kydex will last forver, but I've also seen 20 year old leather sheaths that were in good shape, that werent subject to very hard use.