Depends what you are looking for,
If you do high miles then obviously the TDI but of course these cost more, If only drive occasionally then I would say 1.8T.
I have had both, my 2002 GT TDI I sold for £4650. I then bought a 2000 GTI T for £1850 so as you can see a huge difference which would take years to make up in diesel savings, plus I hated the diesel sound.