Funny you should mention Sweetwater because they're also near the head of the table in my mental pantheon of e-commerce greats.
Their website is honestly just above average. Page-load times are fair to middling, the pages are very busy, vendor-supplied copy takes up much of the item page, and categories are difficult, perhaps just an artifact of the industry they're in. But if you dig deep, there is a lot of detail beneath, and the related-items suggestions are usually at least relevant if not spot-on, which is more than I can say for many sites.
But where they shine is the integration of humans into the process.
Back in 2015, I thought I might pick up a digital piano/keyboard sort of thing. I wasn't having a great time making sense of the lingo, so I gave them a call, and within seconds (I think I might've pushed one button in an IVR system), I was talking to real person who knew everything there is to know about keyboards. We talked for probably ten minutes about what I wanted to do, my budget, the ergonomic and acoustic constraints of my workspace, etc. He explained some terms to help me understand how the offerings differed and how those would impact my playing, and ultimately steered me to one of the models that was already on my short-list. We also talked about companion software, but I elected not to take that step at the time.
I went from hesitant to confident, and completed the purchase minutes later. The cool thing is, that confidence followed me through delivery, unpacking, setup, and my first plinking across the octaves -- I never felt alone like I was just gonna have to figure this out for myself, I _knew_ that this was the right instrument for my needs, and set out through basic finger exercises with the determination of one who's divinely inspired.
Then something even cooler happened: A week after delivery, that same sales engineer emailed me, just checking in. To make sure I got everything set up and was comfortable with it, to see if I could use any further help. As it happens, I was fine, but if something along the line had derailed me, this absolutely would've made a difference in getting me back on track. An expert whose entire job is to make sure I have a good experience with their stuff.
Now, I've had other e-commerce sites email me and follow up, sometimes with phone calls, but it's not the same. In some cases it feels like outright harassment (lookin' at you, Global Industrial). Often it's just a ploy for reviews or further business.
With Sweetwater, it's different somehow. Yeah I'm sure their goal is to keep me as a customer so I'll spend more money with them in the future, sure, and I'm fine with that. But they don't lead with the money, they lead with the caring. And that makes all the difference in the world.
With McMaster it's almost precisely the opposite, I've never spoken to a human there and I don't feel like I'm missing anything at all. They're like the supremely efficient robot that just produces the parts I need, immediately and without fail. There's no need for warm-fuzzies in this process, and they seem to understand that as well.