I personally don't own one, but will weigh in having shot a handful.
The biggest factor is what you want out of it. Is it for home defense, range toy or other? The next choice is barrel length, which will be some what defined by your intended use. The barrel length will also determine what ammo you will need to use. A shorter barrel will give you less feet per second and certain ammos may have issues with fragmentation and expansion.
Otherwise your choices will be mostly cosmetic. A quality weapon light is a must and maybe a Micro Dot.
Have you looked at going with a caliber other than 5.56? 300 blackout is pretty popular and offer a couple of benefits over 5.56, especially if you are considering suppressing it.
A note on AR pistols.... Make sure you know the laws. Do some research if you plan on building your own, as certain seemingly minor things can run you afoul of the NFA.
Sig Braces are selling for dirt cheap at the moment, after the ATF reversed a previous ruling. They are still 100% legal to own and use on a pistol, you just can't shoulder it