Since these rails are pricey I would like your opinions on which to get. The 13 would be lighter but it seems by only 1.2 oz. At the same time, the 15 inch MK14 rail is lighter than the 13 inch mk8 rail.

Does the 15 inch rail have any disadvantages other than 1.2 oz of weight? The purpose of this build would be an all around rifle - can do CQB and long range. I was debating getting the bcm MCMR but I want a rail that is more durable.

Only aesthetics, and that's a personal preference.  I think rails out to the very end of the barrel look like dogshit.  Just my opinion though.  Others like it.

I have a BCM mcmr. I switched from a 13 to 15. The main purpose was that I wanted to mount a bipod and the 13" grip was too short for me. When I held the rifle, my support hand pretty much was right at the end of the handguard. I didn't really want to shift my grip.
Other than that it's pretty much looks, and 13" looks better.

If you're using an AGB you'll have to navigate an extra 2" of rail to adjust it. If you run a WML you may have to compromise when it comes to placement of the light and/or tape switch.

I prefer them as long as possible.