I am occasionally squadded with a Law Enforcement Officer who is also an Army Reserve Rifle Team Shooter and a veteran of Iraq/Afghanistan. He has fired many rounds in combat, and had many rounds fired at him. His personal choice of AR15 is gas piston. That is what he bought for himself. He told me, it is because gas piston keeps the mechanism cleaner. I think all other considerations are a wash.
For a target rifle the direct impingement gas system provides a more accurate mechanism due to less moving parts. But, as my Bud and myself discussed in the pits, when rounds are incoming, keeping that heart rate down is pretty difficult.
If I'm not shooting, I'm reloading.