If you are in the market for a mid-tower dual-chamber case, both the H6 Flow and H9 Flow are solid options. Personally, I like the H6 Flow better for it's form factor and overall design. So why isn't my build inside it? Because I didn't pay attention and bought an AIO that was too thick (38mm rad + 25 mm fans) to fit in the H6, so I compromised and got the H9 so that I could still use the Arctic Liquid Freezer III. Could I have bought slim fans for the AIO? Yes, but Arctic doesn't make any A-RGB 12 mm fans, and I didn't want to return 6 items to keep 1, vs returning 1 to keep 6. The H9 Flow is still a great case; in fact, it has a lot more options for the consumer than the H6 Flow. However, I don't think there's any way to deny that the H6 Flow just looks way better for a PC build in 2024.

Differences
Overall size: H9 Flow is taller and longer by a decent bit, functionally same width.
Storage support: H9 Flow designed for 4 SSDs + 2 HDDs, H6 Flow for 2 SSDs and 1 HDD.
Radiator support: H6 Flow can only have a radiator up top as exhaust (MAKE SURE RAD + FANS IS NOT MORE THAN 55 MM), H9 Flow can have one basically anywhere.
Others: H9 Flow has dust filters, H6 Flow does not. H9 Flow IO is on top of case, H6 Flow is on front panel at bottom.

My build:
3x 120 mm fans - front intake
3x 120 mm fans - top exhaust
360 mm radiator - top exhaust
2x 140 mm fans - bottom intake
1x 120 mm fan - rear exhaust
ATX motherboard
RTX 3080
850W PSU

^^ ALL OF THAT would have fit in the H6 Flow if the radiator was 8 mm thinner (so basically the norm from NZXT, Corsair, etc), which is the most impressive part of all of this to me.

0:00 - Intro
0:27 - Why do I have both?
1:49 - Differences
7:06 - Conclusion