I was trying to decide between the HTM-12 and F-15 myself for a 15x20x10 space.
I didn't go with AT screen because my masking system is not AT. So I was limited
in placement options. I only have 34 inches below the screen.
I went with my simplistic thinking process... Bigger woofer, bigger box, less regret.
So I pulled the trigger on 3 F-15s. This is as big as I can get given the placement
The HTM-12 has been said to perform better than F-15s if placed in a baffle wall. I am
not sure how much difference you would see. Also, things are only "better" or "worse"
when you are AB'ing them. Once they are installed in your room, they become the best.
In other words, I don't think you could go wrong with either the HTM-12 or F-15.
This is only a newbie's opinion. My first build was 4 Volt-10LX as Atmos speakers
for my 5.2.4 set up. They were ordered April 4, received them April 8, and in the ceiling April 9.
F15s then ordered April 17.