The Yamaha version appears to be last years model, and is only 26v, so the Bosch bike is more powerful with 36v.
The voltage makes no difference to the power delivered by the motor under these circumstances. Both motors should be rated the same. The fact that one uses a 36 Volt supply and the other a 26 Volt supply should not sway your decision.
As far as I can see, the battery connects almost directly to the motor unit, as is commonly the case with this type of crank drive. Therefore, additional losses due to the increased Amps required by lower Voltage Yamaha should be minimal.
Having said this, I'd still probably go with the Bosch. It seems to be a more popular unit so is likely to be better supported. Or, have a look at the Kalkhoffs.