I'm looking at this myself now, though with manual orders, not via the Shopify integration.
I have the CustomCat samples here already, Printful are on the way (I ordered them later). They do use different printers. Printful uses Kornit (as does Merch by Amazon). CustomCat uses Aeoon. I haven't seen the Printful samples yet, but the CustomCat prints are quite a bit different from my MBA prints.
The CC prints have brighter more opaque white than MBA, so more "pop," which is cool. But the print feels noticeably thicker/heavier, and the edges of white text looks uneven ... looking closely it seems that a bit of white ink ran outside the edge of each letter into the grove/valley of the weave of the shirt, so it makes an effect similar to a jaggy/pixelated edge, but only on solid white elements, like white text.
Kornit pretreatment works differently than everyone else. It is sprayed on shirt right inside the printer and then printed on while still wet. Everyone else sprays pretreatment in a separate machine and then uses a heat press or tunnel dryer to dry it before sending the shirt to the printer. So Aeoon, Brother, Epson, etc seem to be able to get a more solid, bright (but heavier) white. Of course, much depends upon the settings used when printing, so I'll have to wait and see how the Printful prints come out.
As to my user experience ... CC did make mistakes on my test order, but I am a Beta tester for their upcoming manual ordering system, so I wouldn't count that against them.
Gotta love the prices CC has on shirts like Next Level, but I'm not sure what to think of that unevenness on the white text.
