Historically, Renault have had problems with gearboxes in general, and especially their automatics. I believe things have improved in the last few years, but I'm personally not inclined to trust Renault's auto boxes. If you're buying reasonably new, then that should minimise the risk, but in any case, it's well worth considering an extended warranty.
Diesel will be more economical than petrol and much torquier. In real terms, this means less dropping down through the gears to get decent acceleration, though you'll typically get more oomph out of a petrol engine when you really want some and are prepared to thrash it up to the limiter.
Having driven a couple, I don't think turbodiesels (or possibly turbos in general) go well with auto boxes. There's a knack to driving a turbodiesel auto that involves hitting the gas pedal about a second before you actually want them to start moving. I'm sure you get used to it, but it always bugs the life out of me. With a manual turbodiesel, you just get used to giving it a bit of 'gas' before letting the clutch in - you're still in full control of when the car actually starts to move.
In the reliability stakes, there have been some serious issues with both Renault's 1.9 dCi and to a lesser extent 2.2 dCi diesel engines. These problems have supposedly been sorted in the last couple of years (different grade of
and more frequent
changes) but if you're buying new, you might be best off going for the new all-singing all-dancing 2.0dCi.