I haven't actually tried either bike, but Aldby, who has somehow managed to buy 22 eBikes, sell 19, and eventually kept the 3 he liked had this to say:
Seroiusly, I didn't have 22 at the same time

and now only have 3 electric, my wife's wave frame Agattu, my new diamond frame Agattu and a Wisper 905se which is used by anyone in a hurry. My son borrows one everyday to commute to work, he hasn't got anywhere to store one, so has to walk from his flat to collect one. He actually prefers riding the wave frame Agattu.
Bearing in mind that I haven't ridden either, the consensus I've gleaned from this forum is that the Wisper is likely to be faster (particularly if you don't want to join in and pedal very much yourself), but that the Kalkhoff experience is much closer to that of riding a conventional bike (although faster/easier because of the power assist).
As far as I can tell the Kalkhoff will go up virtually any hill because of the way the gearing works, whilst the Wisper won't manage really really extreme hills. However, the Wisper should go up most hills faster than the Kalkhoff.
It also seems that the Kalkhoff is more pleasant to ride without power (either by choice or by flat battery) since the hub gears on the Wisper are always turning with the wheel, and add a drag factor (or cogging effect).
To summarize: the Kalkhoff offers a better "cycling" experience, but unless you're very fit, the Wisper is likely to get you from A to B faster.
Personally I've got a Wisper on order because I've got a 32 mile round trip commute and therefore consider myself to be in a hurry! I guess it depends on what you want to do with the bike.
Regards,
Elephants