Thanks for these explanations, I used the Item site to calculate deflection of the profile (http://product.item24.co.uk/en/home/...ral-41665.html) for 1200mm and 400N. If I do it right I have a deflection of ~0.01 mm.
I was thinking that Aluminium was a good compromise between weight/strength and offer a good flatness. I don't have enough experience to determine how bad is it.

Sadly, I already have these "light" profiles. Should I go for steel box section instead for X and Y? Do you have any advice plz?