I've previously owned a Gt 1.9 diesel and now own a Gt 3.2V6. The diesel was an absolutely fantastic machine, its obvious why they are the biggest seller in the range. (not just because of the current trend towards diesel power) If you listen to the motor whilst on idle or from the outside they are awful (in a typical diesel sounding way) but on the move actually they sound pretty good. Fuel economy is excellent, I could get 800+ miles out of a tank during an epic drive with the entire family on board. The cabin feels quite spacious even in the rear seats, plus there's the big boot coupled with the split folding rear bench to make them very practical. Gt's look so stylish and are a relatively rare sight on the roads today, which is another bonus.
I only cover about 4,000 miles per annum so after my diesel Gt was written off I decided (after a couple of other Alfas first) "what the hell" I'm gonna have a V6. Now despite my low mileage the fuel consumption was a concern as I didn't like the thought of going on a family holiday and have to re-fuel before I got to my destination. This fear was expunged when we travelled from the midlands to the south coast and back, for a weekend on one tank of petrol. Day to day driving sees 23-27 mpg average which sounds bad but in the real world the V6 is quite an efficient engine, long runs see more like 30-34 mpg. If you currently own a 159 then you will already conditioned towards poor fuel economy as they are heavy beasts. I had a 2.2 Jts and to achieve 40 mpg was a struggle.
The best all round engine for a Gt would have been the light and tuneful 2.0 Twin Spark, but I'm guessing that it didn't meet emissions or Alfa was winding down production of the Twinny and had more 1.8's to shift.
If your wife does low mileage and doesn't tear up the tarmac (like my wife tries to) then I would defiantly consider a 1.8TS, but only after a lengthy test drive, as real world fuel economy wouldn't be that far away from the V6, however power would be a long way off. Like I said earlier the diesel is fantastic and never caused me any problems doing small annual mileage, but they can do.
a good balanced view
I would only add that the JTS is almost certainly the cheapest option in terms of purchase price but if you have 5k+ you can afford to spend the V6 is THE one to have, DNG's is a great option if you are looking for a well fettled one...If I had not spent so much modding my diesel I would sell it and Buy DNG's 3.2 in a heartbeat. with my current yearly mileage the 3.2 makes a lot of sense...it was 30k a year when i bought the derv so it was the only option for me.
The diesel is the most flexible of them all though and whilst it does sound like a bag of spanners at idle it would leave all but the V6 standing in real world application,and my re-mapped 220 BHP example would keep up no doubt and by the time I add the FMIC and the final map tweak and stage two clutch it will be as quick,and it does sound OK once you are on the move.
the 1.8 is as previously mentioned a sweet little engine and if your good lady doesn't want to hare about its a good option, my only concern is that it will cost you 3k+ for a 1.8 TS as they were clovers or blacklines where as the JTS can be had for as little as 1k, a good one for 2k and you would be very unlucky to have to spend £1000 in the first year if you have bought wisely.
The cheapest 1.8 I can find on Autotrader is £3800 and that is a lot of money for an un-tunable car with only 140 BHP and 120 lb ft of torque..the cheapest JTS is around the £1300 mark for what looks a decent one so call it £1100 with haggling and despite its flaws it IS fun to drive.
whenever this topic comes up I am made aware all over again that Alfa accidentally designed its best looking car in two decades in the GT and then failed to furnish it with a decent engine range.
bar the V6 which is not practical for most owners the remaining engines have flaws.
1.8 TS underpowered for the weight of the car and not much torque at all, very peaky, and high oil consumption. short cam belt intervals at 36k
2.0 JTS direct injection woes, high oil consumption and lambda and coil pack failures galore and the propensity to eat their own camshafts for fun. same 36k cam belt as the TS.
1.9 diesel, swirl flaps,EGR,and also all of the normal diesel related stuff, DMF and sounding like a tractor..it's like a supermodel that sounds like Eliza doolittle.
so I advise you to choose the one you enjoy driving the most, and for me that was undoubtedly the diesel once re-mapped....(discounting the V6 of course) i probably have not made your choice any easier!
once again good luck