You may have to search for people who have had both engines to avoid selection bias.
All I can say, trying to avoid bias, is that I picked the 3.0 TFSI because there did seem to be more reported problems here and on other forums with the 2.0 turbo. But, then, there are more 2.0 engines out there. I know nothing about the diesel beyond the emission cheating scandal. I have a good friend with a high mileage A6 with the diesel. He loves it and it has been trouble free for him.
Ultimately, I went with the larger gas engine primarily because the Q5 weighs 4,300 pounds empty and that is a lot of weight to expect a 2 liter engine to handle and give long engine life. It will have to work harder. A 50% in increase in displacement and two more cylinders means a lower work load on all reciprocating parts.
And . . . I admit to being among those who want to safely increase power. It is easier to do with the supercharged 3.0. I went from 272 hp to about 430 with a simple change in one pulley and an ECU reflash. That should tell you how conservative the OEM tune is on the 3.0. That bodes well for a long service life and indicates a lot of headspace for safe power increase, should you want or need to do so. On stock tune the engine isn't even close to being stressed.
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