> And it's probably just me but I find the standard Toyota Prius more comfortable than the black cabs (esp due to engine noise and vibration).

It's true, a lot of the older black cabs were loud and didn't have great suspension. I'm starting to see a few of these (http://www.theelectrictaxi.co.uk/) on the road now, which will at least be much quieter - not sure how the ride is but I'd hope it's better too.