I think if you buy a nice facelift mk2 TT S-line/black edition/special edition (TTS/RS etc) then it will hold value pretty well. It's a car I would like to get in the future but buying used makes a lot more sense with the new model nearly out and I think there are no more mk2 TT. Audi have even removed the info about the mk2 from there website.
One of our neighbours has a 14 plate with one of the new sports packages (contrast spoiler, black front bumper, black alloys etc). Apparently they went in looking at Q3s and ended up with a TT lol. must have got one hell of a deal, I imagine £5-6k audi centre deposit contribution.