I ordered my XC40 in June and for about 3 weeks after that I couldn’t decide whether to stay with my initial choice - a T3 engine, or change to a T4. I ended up having 3 test drives with a T3 engine (the only available car for a test drive) until I finally found a T4 and took it for a drive. Below is partly an opinion that I posted several months ago, but still accurately reflects my opinion.
The power gain between the two is not that significant (it is definitely there though) - in fact I don’t think I would consider T4 over T3 because of power alone (especially the geartronic version). However, refinement is definitely a step up - mainly at low speeds and most noticeably when the car is stopped and the engine is working - I noticed quite a bit of vibration on the T3 and none on the T4. There is also almost no turbo ‘whistle’ on the T4 (which I actually liked on the T3, but for many is annoying) and the T4 it is a slightly louder engine in general.
Mainly because of the refinement aspect, but also resale value, I changed my order to a T4 engine. If budget is a concern, and you are not bothered by a bit more vibrations in low speeds, I think that T3 is definitely a good choice - as good as a three cylinder engine can be. It’s also an important consideration whether you would be using the car mainly in the city or on longer journeys - on the highways you would appreciate the additional power
The question is also what options and how many of them would you have to drop for the T4 engine? Here in Belgium, the T4 increased the overall price significantly - 3000-4000 EUR in price of the car and 1500 EUR in additional taxes because of the emissions. For me it was still worth it though and I ended up keeping all the options I initially selected
Regarding fuel economy - yes, the T3 is more economical, but not by much, also since it is less powerful it needs to be working harder most of the time, which is not great for fuel economy. I would guess in the real world the difference is less than half a litre per 100 km which is negligible.