I've had 3 Mk4 golfs now, started with an S-Plate GTi 1.8T 150, then a W-plate (2000) 1.8T 150, now I've got an 03' 1.8T 180 (I'll post some more about this car later, been meaning to for ages).
This April me and a guy at work swapped cars for the night, he had my 1.8t 150 and I had his V5 150. This car was a W-Reg (2000) model. Generally my 1.8T had a noticable edge in performance, but the V5 does sound really nice, has that V6 like howl. Other plus points on the older V5's compared to older 1.8T are the fact that even with standard options it does have more equipment on board (climate control, auto dipping rear view mirror etc).
Overal I would always go for the 1.8T due to being able to have the ECU upgraded to 200+Bhp for relatively low cost for the performance.