It's a tossup between the two. The velocity difference between 16" and 18" will be no more than about 50fps, so it's a pretty arbitrary difference. The only big thing is that the 16" is going to be louder, but it will be slightly lighter. The gas system is slightly shorter too, so the 18" will have a slightly more mild recoil impulse, but it's almost negligible from what I hear. LaRue reps will tell you that they are ringing steel at a mile with the 16", so it's more than capable.
Wait time is supposedly somewhere in the 4-6 month range right now, but will most likely continue to climb as orders back up in their queue.