ad layout:

Tenkeyless is the "standard" layout wrt both muscle memory from most other keyboards and for example keycap compatibility.

However, I would personally prefer 75%, because the distance to arrow keys or the mouse (if you use a right-handed one) is quite a bit shorter. I also prefer how function keys (F1-F12) are right above the number row and aligned with it.

ad materials:

There are two parts: the case and the plate

Aluminium case is considered fancier, and it's also more rigid than relatively flexible plastic (which can be both good and bad in terms of possible damage). One reason I personally avoid metal cases is the risk of frying the electronics, if they're poorly isolated.

Aluminium plate, however, is less rigid than steel, which might feel/sound better or worse to some people. It's personal preference.