I've test ridden both. With both bikes you have to keep in mind THEY ARE NOT THE SAME AS YOUR ktm, yamaha or other. If you want the same performance as your 250 then buy a 250! But, if you want a green bike no noise, poach single track, ride legally in many areas etc. Then an electric bike is the way to go.
I like the idea of the battery swap with the Zero, that's a great option for not only additional ride time but for maintenance and future battery advances (5 years from now simply buy a better battery). I must have ridden the MX version of the Zero cause the front end had MX forks not the smaller forks, and it had a MX rim and tire. I liked the fact I had the options of switching into easy mode for my wife to ride. The Quantya did have bigger brakes which ultimatly adds to the weight and overall performance but I did not notice any better braking. I guess if they both brake the same why not go with the lighter brakes? Cheaper to replace as well. Overall I liked the Zero due to the power mode options, suspension was equal with the MX version on the Zero, the interchangeable battery and light kit to make it street legal. The light weight was a huge factor when my wife (and me in the single track) rode it.