FALs are big and heavy. Built like a tank. They'll shrug off conditions that will have you screaming for mommy. Good ergonomics, all except the trigger pull.
ARs are more precise. Better sights and triggers, better options for mounting optics. Good ergonomics, all except the charging handle. Pricier magazines, though I believe 308 AR mags are coming down in price, 308 PMags are $20 a pop.
I owned both, sold both, kept a Garand. Sold the AR because the mags were too expensive (they were $80 at the time). Sold the FAL because it was too heavy, and because I couldn't get an acceptable trigger pull from it.
The AR was an Armalite, and it didn't run right when I first got it. Sent it back, they said the chamber was reamed too tight and corrected it. Worked fine after that.
Can I ask why in 7.62? I don't get having oddball AR's, when there is always something else designed for the caliber.
Why not? 308 is bigger and more powerful, and potentially more useful. Might be more convenient too, if you already have rifles in 308.