@oblomov @afreytes IIRC, NTFS is writable from both, and relatively robust.
vfat also works, and that's what I use for USB drives shared between the systems.
I didn't dual boot long enough to find having a shared partition compelling. These days, with the way MS treats shared resources (boot sector, ESP, EFI entries / variables, etc.), I'd be concerned that MS Win would "screw up" any shared filesystem / partition.