Re: GT 2.0 Petrol or 1.9 Diesel?
JTD-M without a doubt. I went from a new Astra 2.0 Turbo to a GT 1.9 and I could not have been happy making that decision. The only point that rings true for me from what people are saying above is the sound: it is different to a petrol, but it's not an unpleasant sound, certainly nothing like a bus or well-driven taxi: these are modern and quiet engines. Once you're on the move you'll not be able to tell the difference sound-wise.
I can honestly say I've not noticed any diesel-type smells at all with my GT either, so don't let the sound or smell aspects put you off. I'm returning an average 25mpg+ more than I was in the Astra (about 70% of my commute is motorway), so the savings become apparently very very quickly...!