I started with the C5 (mostly for my wife) and shortly after got an A9 for myself. After only a month or so, I was wishing for an M1, as the A9 only has the 1 (Omni 8) protocol. Of course Omni 8 is probably the best for most things and most people. But for certain things; sleep issues, depression / anxiety, perhaps Knee issues, brain entrainment, other protocols are better or more appropriate. Also some people find Omni 8 doesn’t work as well for them. So I ended up getting a P9 which hadn’t been available in like a year. The P9 is basically an update of the M1, using the 9 volt battery (like the A9) and around 30 different protocols, plus is has a built-in set of stacked coils, so you can use it without an external coil set plugged in if you need / want Also Bob has a recommendation for LiPO USB C rechargeable 9 volt batteries that have proven thus far to be excellent in use with my A9 & P9. In short, the A9 is amazingly useful and effective for me, but it’s use case is more limited than the M1, P9, C5, B5. Alternatively, you might get an A9 initially and then later get the P9 or other more capable unit. I find having a backup unit very comforting in and of itself.