If you're thinking of using an external usb enclosure, I'd recommend buying one first and testing it out, before buying a case. That way you can choose a different case if the external route doesn't work.
Both cases seem good. I think the cooler master can handle longer and wider video cards than the razer, but being that these are both itx cases, definitely double-check video card measurements and make sure the case can handle it.
I'd avoid water cooling, a proper htpc should be low power with low heat generation, and water cooling pumps would add noise beyond what air cooling produces at these low temp loads.
I'm still confused

by your direction. You're choosing a really cool case, but forcing yourself to use an external drive, and most full size enclosures are pretty ugly. One way around this may be too buy a Buffalo BRUHD-PU3. You can get a pre-flashed friendly one without to much trouble.

- image.png (34.79 KiB) Viewed 28727 times