I'd go with the supported rails. There is always some flex with end-supported rails. Since you are looking at c3 screws, I'd imagine that you are building with very tight tolerances. C3 screws are very expensive. I bought some c5 screws from here, at a very good price.
Since you are building such a high end machine, do you really want round rails? I heard, and I'm no expert, that the Hiwin, NSK, etc square rails are much tighter tolerances.