Rock Auto is great, definitely more consumer oriented and delivery is much slower, seems like professional mechanics rely on same day delivery. The biggest difference I see is that McMaster is curated, you want something there is usually one or maybe a couple options and it doesn't usually openly display the manufacturer. Rock Auto shows you all the options.

Rockauto is catering specifically to auto parts. It is far ahead of their competitors however.

Without RockAuto is right up there with the submitted site.