The Kalkhoff Impulse with it's 15 Ah battery will proabbly give you sufficient range to match that Wisper, but like all the crank drive bikes it has slightly lower power and relies on the motor being able to use the gears for climbing. So it will climb anything, and on the very steepest hills will need less effort contributed from you than the Wisper, but the climb will will be slower in the lower gears of course.
On hills other than the steepest the Wisper is likely to be faster and judging from your experiences,I'd say the journey average speeds of the Kalkhoff could be slightly lower. I don't think the difference will be great though.
Of much more importance is how you find riding that type of crank drive against the hub motor bikes you've had so far. They really are very different so it's important that you try before you buy, since for some this is a "marmite" thing. Whilst pedelec crank drives are popular with many, some really hate them so you really should check that out.