To be honest. If I had been blindfolded then I couldn't have told you whether I was driving a T6 or a T 5 apart from the fact that the T6 was a 150. There was nothing that said to me " I must have that ". Not even the cup holder.
I can see the significant differences going from Nothing to a T6, from a T4 to a T6 or even a T5 to T6, but a T5.1 to a T6. Unless you always want the latest, or your T5.1 has a lot of miles under its belt then, I personally, do not feel there is any difference in the driving experience between the two, certainly not the additional £10 - 20,000 it might cost to change. True you are missing out on ACC, LED road lights etc - if you really want them, but then some of them might be able to be retrofitted, in time. I doubt if ACC will be, but then I like driving, not just steering.
I know I said I was underwhelmed, but you must admit that the way VW introduced the T6, the information and literature available has been appalling. I don't think they really had their heart in it. I know they have had other problems, but even so!