What do you want from it. Comparing to the cars you already have:
GTS to the diesel will be like your M3 to your W12.
One is fast, nimble, fun and sporty.
The other is more comfortable, more luxury, better freeway cruiser and still fast.

So which is more important to you in a fourth car? If you are keeping all your cars, then I would get the GTS.
Burmester is amazing even if you only take advantage of it rarely. I have a Eclass diesel and love it, but it feels so much heavier and less nimble then a regular Eclass. But it is more important to me to get over 600miles to a tank then another nimble sporty family sedan. It still drives great but I understand that it is for comfort and luxury and not fun. I have that box checked with something else.

 

I just drove the Diesel while looking at a 991, I am extremely impressed by the Diesel. Car drove incredible for a Diesel.

 

@W12 What did you end up getting ?

 

drove the regular diesel cayenne in dallas a couple days ago. that dealer is looking for one with my specs. it was very impressive to me. btw, it is hard to not put 20k in options...

 

Actually, I haven't bought anything yet! I got a little side-tracked by Range Rover Sports and SLK's. I'll have an update soon!

 

^^^ i ended up with a panny....