Personally, I think the EBC's are more on the lines of an aftermarket replacement disc. However, technology has come along way since the stock discs were made in the 1970's and the EBC's are made of a better material and are usually lighter than the stock 1970's discs, so it would be an upgrade of sorts.
As far as I know, Brembo and Grimeca (along with other higher quality disc manufacturers) don't have much for the 1970's motorcycles. I assume you are working on a 1970's motorcycle.
81 xs400, although the disk is a shared part number w/ the late seventies xs sr line yamaha. there were aluminum centered and i think drilled, so i figure they're better, but i didn't know if they were enough better to justify the price.
I shouldn't tell you that because then you're competition on ebay, but Yamaha FZR600 front rotors are interchangeable with those EXCEPT you need to add a stack of washers as spacers and longer caliper mounting bolts.