You're basing your decision on the auto from your experience with an Astra ........tell you what, go and drive the TT auto, if you come back dissapointed, then you need to save more.
The auto box in the supra is perfectly suited to the car, it's fast, with perfect ratios to throw your head back in the seat at well into illegal speeds. I've owned both, and would say the auto is better value all day long. The TT6 is more money purely because of it's rarity.
The main appeal of the 6 speed is how ridiculously strong the box is, so modding for mega HP is available. Though judging by your comment of worring about MPG, I'm guessing modding is not on the cards.