I concur many of times I have heard people mention how nice the 21's are, I was even on a OmniDekka coming from Brandon (Nevels Cross) as I didn't want to get on the 21, due to School Services at Durham, and I heard people saying, they are much nicer, spacious buses and that as from buses older than what Arriva puts on. 

But tbh It would be nice to get a hourly service from the 21 whcih terminates at Durham. to have a every 12min Frequency.