I made some comment about this in another thread, but heres my take on it anyway. The original Golf V5 2.3 was 150bhp, same as the original 1.8T. In summer this year a I swapped my 1.8t 150 with my mates v5 150 for the night. I enjoyed the test, but overal we both agreed that the 1.8t is sharper, whereas the 2.3's delivery is more 'cruisy'.
On the plus side for the V5, it does have more standard kit from the factory than a 1.8t, the W reg (2000) one I drove had boot mounted 6CD player, Climate control, centre arm rest and the anti dazzle auto dipping mirror. My previous W reg 2000 1.8t GTi had the 6CD boot changer and the climate, but only because I added them from an options list. Also the V5 has that fantastic sounding howl, very V6-ish, I couldnt resist revving it in neutral at traffic lights to hear it! Haha
It all comes down to what you want, do you make long journeys, need comfort etc?
The 1.8t can be taken from 150 - 200+ BHP quite cheaply, but to tune a 150 2.3 I'm sure it would be much more expensive.
The current V5 is 170bhp, but then the current 1.8T is available at 180bhp, so again it just depends what you want from your car.
I'd always take the 1.8t, for tunability, drivability and the better shocks, recaro seats etc (plus I do a lot of mileage and still find it a comfortable car)