I can't tell you how many times I've walked by the unistruts at lows and said hrmmm I wonder if... I've also checked out the slotted ones too.
Checking out your designs, I'm thinking if you have your rails butted up against each other, It'll be much stronger. It would be like have a steel extruded H beam. Very sturdy. I'm attaching a picture of what I mean. You could drill them every 6 or so inches and bolt them together making them even stronger. Or you could keep it the way it is and drill it every 6 inches and get a some long bolts and nuts and and secure them to each other that way. I played with useing two tubes with bearings running only on the top and bottom and found it was easy to bend them inwards which could lead to racking or would it be chattering.
The way you have your table top supports going underneath the angled tabs, could the unistruts be moved above the front and back cross beams and the tabs used underneath? Maybe you could just get rid of the little tabs all together and just drill some holes and bolt them together. Just seems like there is a lot of stability and strength to be gained from mounting them differently.
Also, how do you plan to adjust your bearings? If you've found a good supply of concentric bushings, please share.
Looking forward to your progress on this build.It's funny we both are building another cnc machine at the same time again. I'm going smaller though .