Sony, although it supports +RW, favors -RW, which is why you can only use -RW for the VR mode. This goes all the way back, for Sony to the RDR-GX7, and continues to this day with the 300, 500 and 900 models.
As for formatting, standalones do not accept blank discs that were formatted or erased elsewhere, like on a PC. This includes the Phillips recorders. Standalones want to format and erase their own discs, and if consumers "tinker" with formatting or erasing discs on a PC, standalones reject the formatting. At least Phillips allows you to "reformat" a "PC-reformatted" disc, but Sony doesn't know what to do, so it just gives you an error or refuses to let you do anything with the disc. Perhaps this is due to the Sony's operating system, which may well be Linux-based (who knows for sure though?).
In addition, the Sony's "dirty disc" error doesn't usually really mean the disc is dirty. You can take the same disc to a PC and it still works. In short, the Sonys are a picky lot when it comes to playing and recording, and still-useable discs are often proclaimed as bad, or as Sony says, "dirty."
So, perhaps the lesson to users here is: Don't reformat or erase your standalone discs on a PC; use the standalone's utilities to reformat or erase. This is not in any of the instruction books, but it should be.