I am already in the process of building a new CNC router. I have a frame and gantry built of welded steel tubing that was designed around SBR20 fully supported round rails. My frame is built out of 4"x8"x0.25" wall rectangular tubing with 2" square legs and some other bracing, its stout and weighs about 620 pounds as it sits, I estimate it will be around 780 when complete. The Gantry without the Z axis is made of 8" square 0.25" wall tubing and weighs in at 180 pounds. I would like to go to 20 or 25mm profiled rails but the cost is prohibitive. I will use profiled rails on the z axis and I am building in such a way that I can upgrade the x and y axis rails in the future. I have found some used THK 15mm rails that are more expensive but I could afford them.
So, ignoring any frame or gantry deflections, would the 15mm THK rails offer much of an improvement over the SBR20 rails?