The ST2 is much faster. If you want to cruise at 28MPH, choose the Stromer. The Kalkhoff Integrale S11 is an amazing bike, too, however, it's better for cruising in the 20-24MPH range. Also, the S11 might climb really steep hills a bit better than the ST2, however, the ST2's additional power (up to 1200W at peak is ~2X the peak power of the Integrale) will allow it to climb milder hills faster than the S11.
I think it comes down to personal preference. I love that the S11 has a front suspension fork standard. The ST2 has an optional fork for $200+. The S11 is probably lighter, t0o.
In my opinion, the Integrale is just a little underpowered. My problem with the ST2 is that it doesn't come with a standard front suspension fork and it's really heavy.