Thought I might be in a good position to reply since I also came from a Mondeo 155 TDCI (although not the ST), which I also loved. There was a brief affair with a turbo petrol in between but I soon missed the "lazy" diesel driving.
I cover around 60 miles/day on fast roads so diesel makes more sense for me. Petrol was fun but I soon tired of having to give it 6000rpm to get any real pull out of it. Having said that, if your long runs are limited to only about 8 times/year, I would seriously consider petrol as I don't think you'll see much economy on the short runs - even in the 1.9 diesel. It's good when warmed up and reasonably
economical cruising at 60ish, but don't expect the economy you got from your Mondeo; less-so of course, if you buy a 2.4
Build quality is way ahead, or maybe I should say "premium feel" is way ahead. The mondeo suffered less squeaks/rattles but getting back into a colleagues ST now, it feels quite cheap/flimsy in comparison. The 159 is built like a tank (maybe a little too heavy). The weighty steering, wide tyres and squat stance inspire a huge amount of confidence on twisty roads and you really have to push it before it understeers.
The diesel is heavy though and requires some encouragement to get it moving - I suspect this is where the economy suffers most, hence not great on stop/start journeys. Once moving though, assuming you've adjusted your driving style to predict and avoid turbo lag, you can make fast progress and I've surprised a few passengers in my time
I wouldn't worry too much about EGR/DPF/DMF issues as they seem to be par for the course on most modern diesels now. If you buy one, get the EGR valve out and give it a good clean straight away (half-hour job - guides on this forum). I got my car at 25k and it was already quite coked up. A colleague bought a 159 at about 80k and found his was almost entirely blocked. I'm repeating the clean-out every 25k.
The boot isn't great - certainly nothing compared to the mondeo hatch, but then practicality probably isn't at the top of your list if you're looking at Alfa's
It's a decent size for luggage etc, but you wont be man-handling any furniture into it, unless you find a rare example with the folding seats option, or buy an SW.
Would I go back to the Mondeo? No. The driver enjoyment and the fact that you're driving something a bit different sets them clearly apart.