I have made this decision in the last month. Having gone from Impreza WRX ((monster (6yrs ago) 230bhp 4wd 23mpg)) to a Jag 2.2d Sport (lots more comfort with only 150bhp, 35mpg) the change was not as big a shock as I expected it to be due to the torque in the mid range.
I therefore automatically wanted the 2.4 for my 10,000miles as on paper it seemed the ideal mix of performance and economy and found a black one at a dealer.
When I went to test drive it they also had a red 1750 which looked gorgeous and having driven both back to back in Meridien Milano I was genuinely imressed with the 1750. I set the trips and over the same test route (15mile mix of town and dual carriageway) the 2.4 returned 29mpg
and the 1750 25mpg - not a great difference imho.
The 1750 performs not unlike a diesel with the sweet range appearing to be between 1500 and 3500 revs but is so much smoother and quieter which was a nice change to the somewhat agricultural Jag engine.
Anyway, to cut a long story short I collected it yesterday and drove from Palmers Hemel Hempstead to South Wales and according to the trip it returned 28mpg at a constant 80-85 (using private roads all the way back of course
I am yet to guage what it is like for my 12 mile commute of 50% A-roads and 50% Dual carriage ways but I am happy I made the right decision (and given the choice you should have a red petrol alfa at some point in your life!)
I think both cars are still at Milanos as a combination of the inflexible attitude of the sales guy and the poor condition of the 2.4 means that the black 2.4 is still there and the red 1750 is not on my drive
I would strongly suggest a back to back drive if you can manage it.
All in all, lets face it you have a nice decision to make