About 5 years ago my old 5hp Genie 20 gal was no longer repairable, I tried to get the part but Genie didn't even acknowledge having sold shop vacs from 15 years ago. I was using it in the basement shop and operating a maze of 2 inch pvc run thru the house walls to various locations. From those ports I would plug about 25 feet of 1-1/2 inch hose to a floor wand.
That old Genie sounded like a jet engine but since it was in the basement, it was ok. But man could it suck! So I finally had to replace it, and with sucking power critical, I opted for a 6.5hp ShopVac. It was amazingly quiet but had no suck!
After that I became very skeptical of hp ratings. After a couple years of using the broken Genie and the new vac siting in the corner I relegated it to the cabin and went looking for another vac.
This time I bought 3 in the $100 range with the intent of testing them and taking the losers back. Since I was using it as a whole house and shop vac system, I required that it function with an internal paper bag in addition to a good pleated cartridge filter that usually stayed clean or caught the fine dust that escaped the bag. Also it had to suck at the far end of the house at the real world end of my floor wand. Noise volume was a secondary concern.
I replaced the floor wand with an empty milk jug and cut a 3 inch hole in it to which I attached a cfm meter and a static pressure manometer, a few tools off my HVAC truck.
I then tested a Rigid, a Craftsman and a ShopVac, all in the same hp and price range.
I no longer remember the specific readings but I remember that the Rigid was very strong, however the design prevented the installation of the internal bag, a deal breaker for me. I ended up with the Craftsman with only a tiny power lead and slightly quieter than the ShopVac.
The ShopVac had clearly changed their design compared to the failed one a few years earlier, and performed just fine. I think the Sears mostly edged it out due to convenience of returning the ShopVac and the cheap 2 Year extended warranty.(I'm one of those fanatics that actually tapes the receipt to the machine in a zip lock bag) A nice feature for a tool that I knew would live a hard life.
The Sears Craftsman performed very well over time, has good real world power and reasonable volume. It is more squat in shape and less likely to tip over when dragged around. I think I like the shape and lay out of the ShopVac better, but it was a toss up.