Well yes, any E-Ink device should be able to open a PDF, but PadMu gives you the ability to sync two devices so you can place them next to each other and display two pages at once. I think it has additional features specifically for working with sheet music, like an infra-red sensor for turning pages by waving your hands in front of the device. I know the Gvido has that (Edit: But the PadMu actually doesn’t; it’s all software enhancements and the dual display mode).
This review showcases the side-by-side display (double mode) feature at around 4:20. Can Onyx devices do that? I haven’t checked, but my guess is no.