Tensioner will shift even after you rotate the engine to check the new belt is OK, this is even more true for newer 1.6/2.0 diesels. Yes, the belt will stretch a little with age, but half of the difference shown could be normal tensioner swing.
I replaced the belt and the water pump on my Roomster 1.6TDI a year ago due to badly leaking water pump, made 3 attempts to set the tension as per factory service manual and always it shifted to different to what I had set after rotating the belt.
Only then I found online a later date factory service manual telling me not to bother about it.
Regarding Dayco vs Gates vs other brands, any kit will do so long as it is undamaged. I ordered a beautiful Dayco belt+all rollers kit from amazon, good price, only on arrival the main tensioner roller had 2 deep scratches on the belt's running surface. Sent it back and went with INA kit and Dayco belt, not so nice but undamaged. 15k miles later still works fine.