If you do a reasonably high mileage then the diesel is the only one to go for.
The 1.6 petrol should give low 40's mpg but is a little underpowered. Don't get me wrong it's a good engine and is more than adequate for most journeys. However, for a more relaxed drive
diesel is the way to go. Rev the petrol hard and it will be ok. Rev the diesel to about 2500rpm and it will fly.
The meganehasn't been without it's faults - what car manufacturer isn't. Early 1.9 dci were more unreliable with turbos failing quite common. AS you are after a 2 to 3 yr old car a kot of the problems on the first models should have been ironed out. Later models appear more reliable.
With the 1.9dci engine the key to reliability is change the
every 9k miles ( not the 18k recommended) and use fully synthetic 5w/40
The 1.5 dci engine will be more economical than the 1.9 and cheaper to tax. For a lot of journeys you may not notice much difference between the two. We had a gran scenic as a courtesy car the other day and thought that it went quite well, not as well as the MEgane , but put it down to being a larger car. It wasn't until we gave it back that I realisd the scenic was the 1.5dci!
If you get a chance drive both cars with both 1.5dci & 1.9dci engines and make up your own mind. If you intend to tow I would go for the 1.9dci.