Hi,
I'd say miles per year is more important.
I did 25K a year in my MK5 GTI and it really felt it after three years. I replaced it with a BMW which has done 85K over 7 years and being honest it feels way better now than the GTI after 75K did. I've got a GTI on order, so no bias....
To answer your question - BMW Diesel - 20K a year, no issues, GTI petrol - I'd probably stick to 15K a year or less.
I've not had a Golf diesel so I can't comment so who knows it could be just as good as the BMW...
Cheers, Neil.