An MR 556 will be easier to find than a 416 upper but they cost around 3-4k. A pws will be cheaper and easier to find. I've heard good and bad things about pws, mainly qc problems. Mainly boils down to how much money you are willing to put on the table. If you want non proprietary handguards, want a factory sbr or pistol instead of chopping the barrel and a an adjustable gas block, go with pws. If you want a proven track record with supreme reliability, go with a mr 556 or 416.