I have both the TA31A (4x, red triangle) and the TA50R-2 (Compact, 3x, red triangle).
The TA31 is great; I have nothing bad to say about it so far. It's mounted on my flattop, and eye relief is fine. The eyepiece does overhang my ARMS #40 BUIS, so I'd have to yank the ACOG to flip it up.
The TA50 (carry handle mount) is on my 20" A2.
Using the standard carry handle hole, it's too far forward for good eye relief. If you want the full field of view, you have to literally take a lip-weld on the charging handle; however, the scope's still useable if you move your head back a bit. I fixed this by drilling a second hole in the carry handle about 1" back from the existing one. Personally I would prefer a bit more optic than the compact ACOGs for a flattop, but one would certainly do the job.
I do like the red triangle: the red is easier to see than the amber, at least for me. The triangle is big enough to easily see, unlike the chevron, and has the benefit of giving you a finer aimpoint unlike the dot. I defintely don't like the donut, those seem to obscure too much of the target for my taste.