That's an interesting debate. My view on the matter is that while at the moment an electric bus may source its power form unsustainable resources, it has something a diesel bus doesn't, which is the potential to run on 100% renewable energy once our grid reaches a stage where it can handle it. A diesel bus cannot be totally sustainable by its nature.
The body is hideous, but the bus is just a trial, and as they're entering a partnership with ADL hopefully we'll see a much nicer looking vehicle come out of it. If the interior is anything like the K-9s, then they really need that partnership *shudders*.
However, I don't think that battery buses are the answer in cities, opportunistic or inductive charging is something I'm much more in favour of. Induction charging is relatively inefficient, but has great potential to be deployed on motorways and the like to keep the bus moving, and charging stations at bus stops would allow rapid recharges whenever necessary. This means a reduction in the size of the battery pack, and therefore a drop in kerb weight and an increase in passenger capacity.