I was sort of considering the same thing.
Just bought a TA31A (triangle) for my M4 and it seems to be a great fit for me.
I heard all the complaints about eye relief and was a little concerned about it, but it turned out not to be a problem. I have the scope mounted all the way back on a Swan sleeve and if you take a cheek weld right where you would on an iron sighted gun (at the charging handle or about an inch or so back) the eye relief is just right. I don't want to have to hunt for the correct cheek weld on each of my AR's, so having a consistent spot to put my face is important.
The triangle reticle is great. The top of it works quite well for precision shots, and it is extremely visible in daylight. Never having even seen an ACOG in the flesh before, I had some vague worries about the triangle obscuring too much of the target. Let me tell you, it isn't a problem out to 100 yards. Past that, things might get dicey, at least until you begin to hold on the hashmarks.
Still, I would like to try a TA01 or TA11D for my 20" flattop. I want the triangle reticle on the next ACOG I buy, though I wouldn't rule out the crosshairs either.