New machine Z axis

So I am doing the design for my new machine and am currently looking at the Z axis.
I had intended to make both the fixed plate and the moving plate both from 30mm thick ali but I dont have enough so it looks like a small design mod is on the cards.
I have enough 30mm thick for one plate and enough 20mm thick for the other one or both.
So going by using the 30mm thick and the 20mm thick whats the opinions on whether the back or front plate, fixed or moving plate, should be the 30mm one? I thought that I would make the moving plate or front one from the 20mm plate seeing as it will have the linear rails fitted to it to improve stability and make the fixed or rear plate from the 30mm thick stuff.
Any ideas either way?

Someone will correct me if I am wrong, but I would say 20mm is enough, so any combination you choose will do.
Personally, I would go with 30mm front plate.

Yea 20 minimum, any lower and your going towards sheet.

.Me

Definately 30mm on Front plate, you'll never bend 20mm on rear but you will bend 30mm plus rails on front if you twat it hard enough.!! I've seen my 20mm plate with rails on bow like banana when shit's hit fan.!!

Thanks guys. Looks like 30mm on the front then.