I noticed that the old DC40's has a DC24 brush motor and the ERP ones use the larger DC41 mk2 motors now which I think is worth mentioning speaking of head changes
40 and 41 ERP models have the same cleaner head. I assumed they had the same DC25 motor in there?
According to Dyson it has 20% more brushbar power than the original DC41 animal so I think it spins at a faster RPM and probably is more reliable , not seen any faulty yet but they have been out a year and a halfish so far