Had all three engines in the 159 and the 3.2 is the best by far. True it is a little thirsty so it all depends on the mileage you do I reckon. I quite liked the 2.2 but that was due to the Mangoletsi Mod 4 which made it sound like a proper petrol engine should. The 2.4 is very torquey and a good engine but in a Spider to me it just doesn't make sense
Don't discount the 2.2, it's not nearly as slow as some people make out, especially when run in. There's no turbo lag and coming from a 1.9 it's not going to feel slow. Plus, even unmodded, the exhaust sounds great on my wife's one (it's an LE but the boxes look standard). Get the Mango's mod 4 and it sounds and goes great - actually now I think about it that would be ace for a spider.
ON the other hand, the Spiders do all have very soft suspension compared to what you are used to, so they may feel slower than your old cars.
2.4 had got lots of peak torque but also understeers a lot more than the 2.2, plus it's not going to sound as good as 2.2 or 3.2.
3.2 would of course be best if you can handle the running costs, but as you alluded, a soft top is never going to be an out and out performance car so it doesn't particularly make sense to get the biggest engine in a Spider.
However, I would recommend you get the underbonnet roll bar thing that connects the front suspension turrets, that will definitely improve the handling on whatever engine you get. It's only about £90.
Just to say, our Brera and Spider 2.2's both get about 27-30 on a run, but around 21-25 in town traffic.