It's the new M2 motor and bigger (but not 900) battery. I just bought the Shuttle LT knowing this was coming. I still don't get the Avinox hype. I've done loads of miles on an AmFlow, and it's a great system, but genuinely and objectively no better than the latest Bosch G5 (G4.1). The power is totally unnecessary if you still want to pedal and work, there's still no range extender on either M1 or M2, and the M1 rattles.
Totally happy with my 21kg Shuttle LT which can be widely adjusted into different bikes, with the reliable, traction-rich, equally light G5 motor. Plus, I switch between 600 and 800 and 800+RE depending on the day out.
The Amp'd has a more handsome silhouette, but other than that, Shuttle LT all day long for me.
Style of bike first, Geometry preference next, Looks third, then the motor now becomes a qualifier, as all the motors are now superb.
Happy trails all