I have an aluminum cased Filco and a plastic cased KBT Race, and honestly, both have their place.
Aluminum (I use an aluminum keyboard for my desktop where it is stationary and not prone to being moved)
Pros:
Gives a VERY solid feel, no twist or flex AT ALL. Very luxurious.
Easy to sand and repaint, have it anodized, or powder coated. I repaint mine every so often.
Doesn't get damaged easily
Cons:
Heavy! I have a glass desk and if dropped from much height, it will destroy it. The Filco/Vortex case combo is 5.5 pounds! It's heavy enough to break your foot if it falls on it.
Often lack feet to raise or lower the case
Can be insanely expensive (used MKC cases can fetch several hundred dollars)
If made crooked or poorly, not only is it not easy to fix, but may not be fixable at all. *
Often you are at the mercy of low volume suppliers
Static electricity... Some thought I was crazy about this, but I watched a 3in static spark jump from my hand to the keyboard case and illuminated the LED's. It didn't hurt the keyboard, but I have since added a wire to ground it to the USB ground just in case. I have yet to see it happen again. Not being grounded means a shock could also come back at you(!), power tools are plastic for a reason. Everything metal that works with electricity in your house is grounded for a reason, even your shower.
Plastic: (I use this on test benches, servers, etc, places where it's likely to be moved or bumped)
Pros
Lightweight
High strength to weight ratio
Wont damage anything if it falls.
Electrically insulated
Often have feet to elevate the case.
Won't break your foot or desk if it falls on them.
Cheap
Cons:
Often poorly made
Offer no structural support
Can feel cheap
Some are molded and then painted, and the paint falls of at a mere touch.
* More than few aluminum cases are made exceptionally poor. Some are cast then milled, if not milled or cast right, it can leave casting marks, some lacked feet when sold, some lacked screws, some had wrongly placed screw holes, some were milled at a slant and some had poor alignment/accuracy. Before you think it's just the cheap ones with problems, high end ones had a batch with bad anodizing, so the upper and lower didn't match.