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I've got the the Ford AIS with the fender sleeve and it's great. It's more pricey then the 6637 but you'll have to deal with less turbo drone then the 6637. If you have any doubt about the turbo drone from the 6637 then you could always PM Kwik. He can tell you all about the 6637 and turbo drone when towing.
But if you don't tow heavy a lot then the 6637 could work out great for you.
Go with the 6637, if you feel it is to loud for you you can go with an AIS, I didn't notice to much of an increase in cab noise unless you really get on it.
I'm a fan of the afe. It has to at least be the proguard filter though. I've run afe intakes and filters for over 7 yrs, and never a problem. They are pricey, but they are also a top quality product.
A lot are happy with the 6637 filter too. Bottom line is don't sacrifice filtration for air flow. Good filtration is critical on these engines.
I also say go with the 6637. I converted my 7.3 Excursion recently and it was a piece of cake. I removed the 4 bolts holding the air box and removed it. I used solvent to clean any oil film or dirt from the rubber boot, slipped it over the 6637 and tightened the clamp. The filter cost me just under $28 (fleet price) at NAPA. I didn't worry about reinstalling the filter indicator because I didn't trust it anyway.
It does make a little more noise, particularly when the engine decellerates quickly. It kind of makes a noise like an air brake unloader on a bus. There's also a little more noise when driving, but I don't notice it unless I'm driving next to something that will reflect the sound like a wall or a truck.