Thanks to bad management (of my headphones), I had to double back on myself this morning to pick them up meaning I got to ride a new StreetDeck, a B5, and the 6 cylinder StreetDeck back to back to back.
I must say, I'm very impressed with the new StreetDecks on the X21 route, they are really comfortable in comparison to the B5s, they also seem a lot quieter. At the speed limiter they're nowhere near as loud as the older batches, and probably actually a bit nicer than the B5s. It did struggle quite a bit on the bank at Coundon, but once it eventually kicked down it was fine.
I still think the seats are a downgrade though, they don't seem to be as soft as the red/blue leather ones, but they are a bit softer than the blue e-leather of the 16 plates.

As for the 6 cylinder StreetDeck, the suspension doesn't seem to be anywhere as near as nice as on the 4 cylinders, it's a lot more crashy and harsh, especially along the A688 at Binchester. It also seemed to be a bit poorly, with the stop/start system refusing to start back up! Every time it kicked in, the driver would have to fully turn off the bus, wait, and restart it to get it going again. One thing that was great about it was how smooth the engine was, at idle you genuinely couldn't tell upstairs that the engine was still running, and even at the speed limiter it was nice and quiet. Much better than it was last time I was on it (which I think was when it was brand new, so probably not broken in properly). I think the biggest compliment I could give it is that I forgot to even pay attention to what it was like!

Basically, despite now quite liking the B5s, if they were to replace them with the new StreetDecks, I think it would be alright, but I really would prefer if they swapped the old interiors over!