STS (as a German, I am having a hard time with the SS) is great if you like a very stiff, solid feel of the switch mount. I love it, you can really HACK on it and there is literally no impression beyond the keycap travel.
Alu offers more colors and flexes a little more. That "little more" does give the keyboard a VERY slightly different characteristic, letting your fingers land just a tiny bit softer when hacking at it.
If you're the guy clipping stabs and hacking caps ... the SS STS option is for you.