For me it was the mainly the cab, it's odd really, where do we spend most of the time? But for me, I liked the T6 cab, it set it apart from, say a conversion, or theTransporter, it was more like driving a car.
I'm very disappointed VW made this change and for me it was an unnecessary, change for cost saving (IMO).
I didn't initially want to mention it on here in case the new owner was on here (although I'm sure a lot of the issues have been sorted now), one of the main reason for selling it (I bought it brand new), it has so many problems, the final straw I think - apart from the constant engine warning lamps, catalytic converter needing replacing, 2nd leisure battery needing replacing amongst many other little niggles which were covered under warranty, was the alarm constantly going off, I can't remember how many times I took this to the dealer. Eventually they found a fault in part of the wiring loom and replaced it (the wiring loom), but the damage was done for me and I just wanted to see the back of it. Haha of course I now regret that decision and wish I had stuck with it as I know all issues can be resolved and better to get them all done under warranty.
As a balance, here is what I thought was better in the T6.1 over the T6 (some maybe my vehicle specific) My 150 BHP T6 used to often lurch off from a start and even wheel spin (many others haven't had this issue?, the T6.1 was much smoother, oddly, the side door on the 6.1 was much smoother! The bed setups and also the rear 'parcel shelf' on the 6.1 was better, however, all of these could be retro-fitted into the 6.
But, as
@Resfeber said above, having one new and ) % finance and warranty is good and of course with all vehicles - age + wear ant things needing replacing.