I recently bought my car, and tried a standard 1.9, a chipped 1.9 (Mangoletsi) and a standard 2.4 in the process. I found the standard 1.9 to be acceptable, but not exciting to drive.
The performance of the chipped 1.9 and the standard 2.4 seemed very similar; it is a noticeable upgrade in either case over the standard 1.9. However, the chipped 1.9 doesn't sound as good as the 2.4, and I don't know if this is important for you but I much prefer the look of the twin exhausts that you get on the 2.4!
In the end I opted for the 2.4; the price difference between the two was not great anyway, once you factor in the upgrade cost. I am also a cautious type, and feel safer having a standard car than a chipped one; I'd rather be running a car that's operating well within its limits than one that's being pushed to its limits, but that's just me.
As for the consumption, I've only got my 2.4 to go on, but it is very variable depending on the road conditions and how you drive it. If I drive carefully I usually get around 32-35 around town, and 37-42 on motorways. If I am giving it some, or if the traffic is stop-start or very short trips, it can be more like 25-35 around town and 34-38 on motorways. I've found that putting the trip computer on to show current consumption as you're driving helps you learn how to adapt your style to get better fuel efficiency.
I think the main reason the car feels as if it's not giving great consumption sometimes is because it accelerates quite effortlessly, so you're not as aware of just what you're asking the laws of physics to do! I.e. going from 50-80mph takes no effort, but obviously requires some fuel to make it happen!
I definitely don't regret going for the 2.4. I think I would have always wondered what I missed out on if I went for the 1.9.