I have just about every combination of plastic and metal you can get and I can safely say that the only simple, obvious things about metal cases are that they flex less and cost more, everything else about them has a trade off.
Don't get me wrong, I like aluminum cases, I just think they are a bit overrated.
Here are some of the things that many either don't know or consider before getting an aluminum case.
Durability - While stronger, if you do scratch the anodizing or paint (quite a few have had flawed finish from the factory) it will be very obvious and not easy/cheap to fix properly, anodizing can cost as much as the case. Whereas plastic can take a scratch or impact and often not show it as it's solid colored throughout. Plastic can also absorb impacts saving your pcb and switches and it doesn't transfer impacts to your desk (more on this in a second). Paint and anodizing can wear off over time as well.
Weight - We equate weight with quality FAR, FAR too much. While it does make the keyboard sit more stable, if you drop it it can damage what it falls on, a Vortex Filco case has enough weight (about 5 pounds!) to easily break a toe or foot, also that increased mass means it may hurt itself (switches, caps) in the fall. If you have a glass desk or small kids be extra vigilant, I've come come close to breaking my glass desk and toes a few times, it's easier than you think. It's also much easier to carry a 2 pound keyboard than a 5 pound behemoth if you plan on taking it anywhere.
More luxurious feel - Highly over-rated. It can feel warmer or cooler than room temp (good and bad) and machined cases can have sharper edges or slivers that were not properly cleaned during manufacturing. Injection molding rarely suffers this problem, and unlike plastic you can't simply shave that sharp edge off without damaging the finish.
Noise - Yes, aluminum keeps lower/lighter sounds/impacts from reverberating through the frame however, it can also reflect key activation sounds back at you, as well as transfer heavier impacts right into your desk. My Filco is quiet at high frequencies like key activation (after adding foam to stop reflection) and at small to minor impacts like bottoming out, but harder key impacts transfer through the case and feet and right into my desk, the KBT Race on the other hand (not even a metal plate), you could practically punch it and it would hardly send a shockwave into the desk.
Remember,
Weight and sound dampening is easy to add, frame stiffness is not and that is what you actually will notice more than anything while typing, typing on a wet noodle sucks (KBT Race). Were I to do another full custom keyboard (and I may very soon), it would be be a carbon fiber or (molded) polycarbonate case (painted internally) with a carbon fiber, aluminum or titanium plate, this way I get all the stiffness and durability without all of the weight, then add foam inside and if necessary, some brass or lead to adjust the weight if I feel it needs some.
