To be fair, I don't know myself. I'm just regurgitating gearslutz brand/model superiority aphorisms. Among the higher compliments the UFX was paid is that some people think it's more comparable to Apogee/Prism/Mytek/Lavry/Super-big-boy converters than previous RME gear, but that said super-big-boy converters are still better.
As always: the 'I read that on gearslutz' caveat
Drew, as I understand it the 2626 is a good unit. I haven't used it, but I've often seen it compared to the saffire pro40 - I've got one of those, and it's great. It's not RME great, but it's great enough that it's closer to the sound quality of the fireface 800 (that cost 3 times as much) than the 800 is to the UFX. I suspect there wouldn't be a really appreciable difference unless you got something way more expensive.
Besides, while I love good sound and device capabilities that are basically audio god-mode as much as the next guy, the most important thing is reliability. That's what compelled me to use RME in the first place, and ~7 years of my 800 working absolutely perfectly, every single second (seriously, it's not even had one dropout), that's why I got another when I wanted more bells and whistles.