my 2p for what it's worth - I have an oxygen hub drive, a friend has an impulse 2 Kalkhoff, I've used both enough to know the difference. on a sunny day the SS system on the oxygen provides a kind of wave of torque that over longer distances make me work harder (unless I'm going to work and sweating is not good), averaging 25 to 30 mph and arriving at a destination after 15 miles or so thoroughly but wonderfully exhausted. If it's a rainy day and I need to get to work on a grim morning dipping below 15 mph on max assistance means not having to do much (which is a relief). on the kalkhoff everything is much slower (25 to 30mph is not possible), dipping below 15 mph isn't ever quite effort free. I suggest the OP try both. you lot are comparing apples and pears - we all have different unique requirements. choosing again, I would go for a SS - but I might choose differently if it were a 27 mph endeavour, but probably wouldn't because id have to justify spending £3k instead of £500 so it would have to be a lot better, when im really happy with the oxygen and not sure if it would feel better at all. Moral of the story is its all qualitative and personal (money aside)