I currently run a '96 A4 quattro 2.6 sport. Very happy with it, MPG is reasonable, and it goes well enough. Slightly slower 0-60mph than a stock 1.8T with the same power (150PS) but has more midrange. Max speed is similar.
Personally, I'm not a fan of 4 cylinder engines, I prefer the smoothness of a V6 / V8. 4 cylinder engines can sound coarse at high rpm too.
1.8T engine is obviously more tuneable, being forced induction. But, there are supercharger kits available for the V6 engines, if you're handy with the spanners, there are Eaton blowers on ebay for not too much dough too, so the choice is yours.