We regularly get near 50mpg out of our 1.9dci - that is for mixed non-motorway driving.
We test drove a 2.2dci - went well. But didn't think the xtra power was warranted as it was more expensive to buy,insure and put fuel in. if you are towing a shed ( caravan) then the 2.2dci will probably be worth the extra.
On a decent motorway run the economy of the two engines will be comparable,but if you do more town driving then the economy of the 2.2dci will be slightly worse.
The main thing to watch out for is that the 1.9dci must have fully synthetic
and be changed at least every 9k miles - the 18k recommended by Renault is too long and causes problems with the 1.9vengines. Mind you I would still do the same for a 2.2dci.
If you get a chance test drive both cars, preferably on the same day. Make up your mind and see which you think is the better engine.It is your money and only you can decide. People will give their honest opinions but at the end of the day choos the vehicle you are happy with.
I would buy a warranty for the non- garage supplied vehicle as repairs can be expensive, also make sure the garage supplied vehicle comes with at least 12mths warranty.