Those were my two choices, so here's my thoughts...
This being Ireland, the 2.0 diesel will be easier to sell when you're finished with it, but that does assume that diesel and petrol prices stay the way they are, which is not guaranteed, and in any case the residual (in Euros) isn't going to be as much as the difference in purchase price. If you do the mileage (>20,000km/yr) then go for the diesel, but I wouldn't get it just because "it's more efficient".
I haven't driven the 2.0 diesel (not for want of trying, but nobody seems to have a demo car with this engine). I did drive the smaller 1.6, though: If you're really concerned about residual value and suchlike, then the 1.6 is what you should be looking at, not the 2.0. The only downside to this car is that it's a bit clattery at idle - something that I've heard said about the 2.0 too. When on the road, the engine note actually gets smoother and much less intrusive. In this, it's the exact opposite of VW's common-rail diesels.
The 1.4 MultiAir (170) is a beautiful engine, there's really nothing like it at the size. This would be my pick in a manual, without a doubt, and that's even with the current Irish anti-petrol bias.
With a TCT, my head says the diesel should be the better drive, but I haven't driven either TCT model, because there are none in Ireland, and it looks like there won't be (don't bother asking, you won't get an answer. I've been trying for a long time, and have finally given up). If you did want one, it would be possible to privately-import from the UK, and I'm sure the dealers are fed up enough with Alfa Ireland to help you do this.
(By the way, for UK-based onlookers, the QV is a non-runner here, as it's taxed out of sensibility in Ireland - €35,000 for the car, plus another €650 a year in road tax. Compare with €28000 and €160/year for the MultiAir, or €23000 and €110/year for the 1.6 diesel).
So what have I decided?
Sadly, it's "spend the money putting a kitchen in the house, and see if they facelift the Giulietta in 2014"