I used the Nuvinci + Bosch Performance combo for a month (replacement bike) and have also used the Bosch Performance drive with a cassette when I swapped e-bikes for a weekend with a colleague. Both e-bikes were 28mph iterations. The N380 bike was a Swiss made Flyer, whereas the 'cassette bike' was a Haibike Trekking Xduro S 5.0.
To answer your question, the cassette has a wider range of gears. I'll spare you the ratio math and simply narrate my experience. With the Nuvinci N380 I had trouble climbing hills above 12% grade and I even managed to stall a couple of times around 15% grade. With the cassette driven Haibike, I climbed an average 18% grade hill containing a short but steeper 22% grade section. It was tough but still doable. I weigh 220 pounds and am moderately fit.
I could not tell you exactly at what percentage grade you will start having trouble with the N380 because that depends on your fitness level and weight. But if you frequently need to climb hills above 12%, I would recommend staying with the cassette. Otherwise the N380 iteration will be more a lot more fun to ride.
A tip if you get the N380: you can easily shift to a lower gear ratio under load going uphill. But it's sometimes more difficult to twist 'to a higher gear' when gaining speed. The trick is to back off pedalling for a second whilst you shift. It makes the experience a lot easier on the hands if you tend to get blisters. This was the main issue I encountered.
