I usually use Amazon for their free and prompt delivery.
I use both for vehicle parts. I am waiting on an order for Camry struts to replace the fronts on a '98 w/275,000+ miles. Rears already done. The price for loaded struts from Rock Auto is about the same as local pricing from a local bricks & mortar chain, for 'no-coil springs' struts. A local light-repairs garage wanted $1,600 to replace just the rears on the Camry! "Uhh, ok, I'll be back-in-touch!" Not for that price!
One thing I don't like about Rock Auto, is that the system usually will-not accept my 1st or 2nd attempts at payment. Yes, I fill-in the credit card info, phone # and the address, no I don't have an account. I do 'guest,' as I suspect there is less of a chance of a hacker getting my info if I
do-not have an account. After I hit 'submit order,' I wait a few minutes and check my email to see if I got a confirmation. If I cannot find a confirmation email, I enter the info again. Yes, I
know that you're supposed to get a page after the order is submitted, with an order # and a confirmation of the cart items. If I don't see that page, that's when I go to my email and do a search for my Rock Auto order history. If I don't see my order, I return to the cart/basket contents page, and do a re-submittal. Yes to searching for yellow highlighted blanks on the order form, where you need to supply additional information before the order is accepted for processing. That's about the only RA gripe I have.