As for the sides, I'm not sure whether to go for 15mm or 20mm thick plate. Price wise there's not too much difference, but will there be much difference in stiffness?
the gantry sideds?.... 20mm! even if the flex isnt much better the added weigh will help dampen resonance
im convinced every single bit of weight and stiffness you can get will earn its keep over and over
i guess the weight has to be watched if you want good acceleration but id go for quality of cut over speed if i had a choice