Check out my post
http://www.cnczone.com/forums/showthread.php?t=46326 . I Just got done building somthing similar to the one on the left. I originaly had one like the one on the right using 1/2" Aluminum and I had some flex, but only when moving it with my hand. I didn't notice any flex while cutting, but then again I've only been cutting for a few days and take small slow cuts. But I wanted something more rigid and the new design is solid, very solid. I would suggest you use supported rails as I did that way the main plate is super rigid.