If i was buying secondhand and it seems to be what you started the thread out with.
GTI i would look for cloth.
GTD i would look for cloth (I like it better than the GTIs as its like the series 2 Mk1s in colours)
R i would look for leather.
Alcantara i hate the stuff. It loves oil and greese and is not that long lasting you just have to look at some 10 year old cars with it.
Anyone else go look at the cloth/leather samples on there dealers wall and notice how much thicker the mk6 weave is compared to the mk5.