I've got three shops set up - Gumroad, Society6, and TeePublic (a little excessive, but I'm still working on it).
My Gumroad shop is for all the milestone rewards that patrons may not want to pledge so much a month to get. So instead of pledging $3/month to get an avatar pack, they can pay $3 upfront to get the download file for it. It sounds redundant, but it's actually a good value to just pledge - once you do $3/month, as long as you stay at that monthly rate and don't go down to $1 (higher is fine tho lol) then you get all future packs as well; vs. if you buy the avatar pack, it's only version 1, and if you want any later versions, you'll either have to pledge or pay another $3 for the new pack. Same goes for wallpapers and such that are for sale - pledging works better for the long term, but if you want just one specific wallpaper or pack or whatever it is, and don't want to dedicate months of pledging, it works.
Society6 is for my prints and TeePublic are for my t-shirts, that's pretty much it.
I haven't personally found any results with any of these sites yet, but that's because of my specific situation - I don't offer a lot of prints and t-shirt designs and pledge rewards for sale yet, so there's not a lot to pick from. Plus my following isn't that large when you compare it to those who have successful stores (and my readership is filled with a lot of younger viewers who don't have much money to throw around; this is just something that happened lol)
I'm hoping that when I have more designs and products to sell and gain a bigger following, I'll start seeing more sales. Might even get rid of my TeePublic store and just sell all designs on Society6 (as they do tee's as well, but I don't have any experience with the quality of their apparel products; whereas I own like 2 TeePublic shirts and they're both awesome lol). It just depends where I get the most traffic and sales when they start to (hopefully) take off. I definitely like the possible benefits of using Gumroad so far, seeing as how I've purchased things from artists myself on it (I've bought a few of Sakimi-chan's tutorials and PSD's and brushes, it works for me because I just don't want to pledge $10-$20 a month for every tutorial when only a few of them actually catch my eye).
That's my two cents! It's definitely something you have to experiment with, find results with, and once you do, you can narrow your choices from there, figure out what decisions make sense from a business point of view, etc etc.
/thumbs up