Couple of reasons - firstly, the board is limited to 17bps on an Ion, it's the Millennium/PSP/PA that impose a 15bps cap.
Also the Z-board egg will *just* feed 15bps on-gun (Forget the 22bps drop test rubbish - less accurate than Gareth Sykes!), whereas a Halo/Reloader B will sustain 18/19 with ease, giving you a little in reserve.
Also, Halos are stronger and have (IMO) better balance - esp. for a lightweight uzi like the Ion.
Also the Z-board egg will *just* feed 15bps on-gun (Forget the 22bps drop test rubbish - less accurate than Gareth Sykes!), whereas a Halo/Reloader B will sustain 18/19 with ease, giving you a little in reserve.
Also, Halos are stronger and have (IMO) better balance - esp. for a lightweight uzi like the Ion.