It seems to me that the writing is on the wall for 15s, my guess is there will be a niche selection of tires available in the not-too-distant future (but that's mostly just a guess). Since I planned on replacing my wheels eventually with beadlocks I am making my future plans based on that view, however I fortunately don't have to decide for at least another year or so and could be proven wrong by time I have the budget for new wheels anyway.
I don't really put much value in the C vs E sidewall discussion, it seems overblown to me. It's probably there - but that's not what I am focused on. My shocks can be tuned to deal with some decent amount of that concern.
The 15" has plenty of stopping power, the difference is slight. Fortunately the difference between the two is just the rotor (fairly inexpensive) as the knuckle is dual drilled/tapped where you can adjust the caliper saddle for either with no other changes.