Never heard of the mac... but have put DIY 6637 intakes/filters on all my trucks, and my buddys. Highly recomend them, why pay big bucks for a name when you can get a great HUGE filter for less?
And haven't heard enough good anything about K&N to ever put one on my trucks, heard too much bad stuff... seems to be mostly related to the drop-in filters made by them for the stock air boxes, but if you scrap the stock air box anyway, just go to the 6637... huge filter, cost about $35-$45 once a year. It's less cost than an oil change, and you do how meny of thoes a year? I do about 10... between 4 trucks, all 7.3 PSD's, so the air filter factor isn't much in the maintnance budget.