NeddogeMax storage:
XDUOO 2 microSD slots for a max of 256G (I have heard success with 2x 256G cards for 512G)
FIIO 1 microSD with a max of 64G
also, rockbox adds a lot of additional functionality for the XDUOO
I own both, my wife likes the color screen on the FIIO for displaying album covers etc, I prefer the extra storage (lossless (FLAC) files can get pretty large (close to 1G for a full album))
NeddogeWhen I was looking a couple of years ago, I settled on the M3 because the M3 was smaller and much thinner...............I use it mainly for outside use, on the go. The M3 is also lanyard capable another plus for me. I have been satisfied with the M3, but admit that I can't compare performance with the X3 since I have not heard the X3. You may have different priorities than me.