Frankly, I'm of the school of thought where the POWER SUPPLY and BUILD reign supreme. Case ergonomics are one thing...but if you've bought a case that has a power capacity that's near or at the max current load for its P/S, you're utterly screwed.
Let's compare two VERY different 2 x 104s...the Mantis, which we're dealing with here, and Pittsburgh's EP-208.
On first glance, the choice seems totally obvious: the Mantis retails at USD 335, the EP-208 at 699. Buy the Mantis and YAAAAAAYYYYcheepnizz...
Except...
Let's lift the hood on these. The Mantis offers a power supply with a whopping 3A on the +12V rail. But...look more closely, and this is a distributed 3A; each section for the +12V rail has a max capacity of 1A. The remaining rails are pretty pedestrian: 1.1A on the -12V rail, and only 300 mA on the 5V. You get 36 bus connectors, threaded rails, plastic housing with stacking brackets. Max depth is claimed at 61mm, but really is more like 55mm so that you have room for power ribbons.
Now, the more expensive EP-208. In this case, you get 4A on the +12V, 3A on the -12V and 2A on the 5V rails. No "zones", either; if you need all 4A on one header, you got it. 40 connectors here, sliding nuts, a beefy WOOD housing with carry handle and lid, and a maximum depth potential of 107 mm, but really most are around 90mm. Either way, the depth capabilities trash the Mantis, as do the power capacities.
Now...let's have some fun! Let's load BOTH cases with about $4k worth of modules, hand them to you, and say "You need to take ONE of these and smack it against the wall REALLY EFFIN' HARD, and whatever's left, YOU GET!" I don't think you'll want to try this with the Mantis, frankly...but from my experience with wood-cased synths, whatever's in the Pitt should be relatively OK.
Basically, it's NOT JUST about the power. That's important, sure, but once you've found your suitable current over-spec (and you should ALWAYS majorly over-spec power...more amperage is always better than almost not enough!), take what's in the case into account and go for durability. Ample INNER space is good for heat dissipation, and that's good for all of your build's components. Rugged housings protect against ANY impact...and it doesn't matter to your synth if that impact comes during a load-in at a gig or if your cab slips off your worktable.
Sure, I think it's great that these cheaper cases ARE bringing down Eurorack cab prices at the lower end, as those prices were approaching total psychosis just 2-3 years ago. But there's a big difference between a cheaper case and a compromised case, and while some users would be fine with a plastic case like the Mantis, the minute it's out of your studio, it's in a certain degree of danger...along with its contents. Keep that in mind...