My comparisons aren’t based on anything scientific Rene, just my first hand experiences. We’ve had four 2.0i Tiguan diesels and four 2.0i BMW diesels (3 and 5 Series), often at the same time, so I’m not just contrasting two specific cars (I’ve had 3.0i, 3.0d and 4.4i BMW’s as well, but not relevant to this). Whilst the power and torque outputs might be near identical on paper, in reality I find the BMW engines to be quieter and more refined with better performance and better fuel economy than their VW counterparts. There was a definite lag with our last Tiguan (the only 190PS one) although I accept that might have been an issue with the car, although when I did a bit of research on a Tiguan forum others reported the same and said that it was unique to the 190, with 150PS drivers not reporting the same issue (indeed, our previous 140 & 150PS Tiguans didn’t have such noticeable lag). I’ve never experienced any perceivable lag in any BMW diesels. If it’s there, it’s absolutely minimal in comparison.
The X1 does indeed have a larger fuel tank, which is a godsend. As I have said, we didn’t choose it for fuel economy, but I’ve been pleased to get 44mpg average in the summer and 40mpg in the winter. I agree that’s nothing spectacular in this day and age, but it’s considerably better than many other 2.0i petrol cars that have been on the scheme. On paper, as you say the Tiguan diesel should return better mpg but we only averaged 38mpg during our 3 years of ownership.