Many things effect how a case will feel, however, how much, depends on the person, for me, a stainless steel plate feels different from an aluminum plate.
Personally,
I prefer aluminum, it feels good and it lasts, however, I also really like my Magicforce which is a plastic case and aluminum plate, it works really well. Better yet, it's so small and lightwieght that I can literally toss it onto a shelf, the floor, a shelf etc.. and not worry about it hurting anything. Compare that to my Filco, which I'm sure has put a few dents in the floor under the carpet. I don't dare let it fall onto my desk or toes (it weighs 5 pounds!). So which is better? Well I recently bought a new 75% and it's aluminum, but it was almost a plex case, why? Because so long as it's stiff enough, it won't matter. The main reason I didn't get the plex was because I didn't like how the case was held together. It was better constructed, and the company had a good reputation, but I didn't like how it was assembled.
What should you get?
Key switches are FAR more important than the case construction, find the switch you like, then later, find a case you want to put those switches into. You type on the keys, not the case, so long as the case is decent, it will be good enough.