On Wed, 16 Jan 2019 12:29:53 GMT, DerbyBorn
wrote:

Are all the different models basically the same motor / battery / drive
units with just the odd gimmick and different heads added - or is there a
step change somewhere in the range?

I would say to make sure you get one with a lithium ion battery. Maybe
they all have now, but it is vastly superior to the previous NiMH.