The R5 has the better balance. It is also quieter because it is composed of thicker metal. It is entirely up to you to make your decision. They both feature good ventilation and cooling, so don’t be put off by their tightness and sound dampening.
The R6 is more modern, but the R5 is my favorite because it is constructed like a tank.
I have a definition R5 for my NAS and can attest that it is well-built and can house a shitload of drives. I currently have 16 of these (slightly modified) inside. While the R6 is unquestionably flashier and presumably more adjustable, I believe you would not be unhappy with either.
The only minor concern is that the thumb screws are difficult to remove. The R5’s front bezel can rattle a lot.
R6 is a bit larger, so if you wanted something bigger, you should have gone with that, and while it will support E-ATX, it appears to be poorer for water cooling.
Do you require both of these excellent cases? E-Atx is the main factor. The R5 has a pretty ineffective three-speed fan controller, but the R6 has a true PMW fan hub.
The 7 is slightly larger, but the most significant change is that the 7 can optionally hold a lot more disks. The 7 may be changed to a “storage layout” and can accommodate up to 14 3.5″ disks at the expense of less GPU clearance. This necessitates the purchase of a large number of additional hard drive brackets.
ALSO SEE: Fractal Design Define R5 Vs 7 Review
Out of the box, however, they are nearly functionally and physically identical: same connectors, same 6 3.5 “Drives, same two SSDs, same fan configuration, and roughly the same GPU clearance.
If, like most people, you’ll never stuff 14 drives into a modern ATX chassis, go ahead and get whichever one is the cheapest when you’re shopping.