As someone who has driven Streetdecks and B9's (native and otherwise), Streetdecks seem more suited to local start stop work which I would argue the 56 sort of is as there is no real high speed non stop running sections. The B9's when allocated to my scholars work often used to overheat after the high speed non stop running (the Voyager ones however seemed to be ok). I would say, as a driver, that Arrivas DB3000 Geminis are a nicer drive and a lot smoother than a Streetdeck though.

Overall, a B9 you have a big engine with a fair amount of grunt not doing very much whereas the current Streetdeck is a small engine working very hard. I would say that the newer Streetdeck with the new engine has scope to be a significant improvement on the original version.