Well both use DDR3, both use Intel i211 NICs. So the main difference is the processor. i7s are superior to those available for the Fitlet, even though the i7s may not be the current generation; it is a pretty device though.I've got two Qotom's one for test and one that runs live, one i5 and one i7, the i5 runs live; neither have ever given me a problem and I know of may others who have them and have had zero issues. At the end of the day either one will work well, so really it's a question of personal taste.
Dual mem slots board:https://www.aliexpress.com/item/4000738202595.html?spm=a2g0s.8937460.0.0.643c2e0e0FyWZD
With case:https://www.aliexpress.com/item/4000893344365.html?spm=a2g0s.8937460.0.0.643c2e0eAwa3ey
got a similar qotom box ~1year ago and it´s been working without problemsi got the 7500u 16gb/128gbi also added a usb driven fan that i attached on top of it, maybe not needed but it does run a bit cooleridle about 50c