I've replaced both pullies as well as the tensioner, belt and alternator. The squeaking continues and gets loader as I accelerate. When idle the squeaking stops. Is there a difference between belt from dealer vs aftermarket?
Dealer (OEM) vs aftermarket parts has always been a big debate.
To answer this depends on the part and the manufacture. Some manufacturers make parts for the auto manufactures. When it comes to belts, Gates is a huge supplier to both OEM and aftermarket.
Aftermarket parts are less expensive and are supposed to be made to OEM specs. Technically, you should be getting a good quality part. Obviously, this may not always be the case.
In some cases, the aftermarket parts are better than OEM because they have the chance to work out the original flaws and improve on the design.
Here is an informative article - Aftermarket Versus Manufacturer Car Parts.