Like you Flud I test rode both the Bosch and Yamaha haibikes off road and preferred the Yamaha. The Yamaha motor felt slightly stronger and I preferred the way the power was there straight away with no delay. On paper at that time the Yamaha had a bit more torque than the bosch. I found the Yamaha system a more intuitive way to ride, particularly off road, as it gives you more sensitive control. You can adjust the power you put down very precisely with your legs. The lack of delay actually makes it easier to control in slippery conditions as you can add or remove power very quickly and with immediate control through your legs.
However my bike is a sDuro that I bought almost exactly a year ago and since then I thought Bosch, perhaps in response to the Yamaha competition had reduced or removed the delay. I think on road a small delay is of no consequence, but off road the lack of delay gives you better control of the power you want to put down.
However when I test road both systems in March 2015, they were both a revalation and very good with nothing much between them. I was choosing between the Haibike hardtails at the time and the bosch one was £500 more expensive, so I would be lying if I said that price did not come into it as I wanted the bike to perform a specific task to enable me to commute to work and leave the car at home as much as possible.
I am sure I would have been pleased with either bike but I am still delighted with the sDuro after a year of ownership and 2500 miles. To have two such good systems available can only be a good thing for the consumer as it brings more choice and keeps both manufacturers on their mettle and this should lead to improvements to both systems.