To cut a long explanation shorter, i'd say it depends on the mileage you do a year and how far your average daily commute is.
A 1.7 CDTI will achieve between 50-55 mpg, BUT that is only when the engine is fully warmed up and you are going about 60 mph in 5th or 6th gear. In towns, cold engine and stop start traffic, it is likely to be almost half that.
A 1.4 petrol will do between 35- 40mpg which doesn't sound as good, but petrols don't take so long to warm up as diesels meaning they get to the optimum efficiency far sooner. Plus you probably brought the car cheaper than the diesel version.
Plus, modern diesels do not like short journeys. They have so many valves that can stick shut or stick wide open meaning you'd get eventually get loss of power, stalling, and engine management lights on the dashboard. Its recommend if you do do a lot of short journeys, to balance it out with longer journeys where you can rev the nuts off of the engine to unstick these valves.
Another thing to consider is Road fund license. The diesel one will be far cheaper to tax than the petrol down to lower CO2 emissions over the petrol.
I'd recommend driving both as well. If you are used to a petrol you'll find its a smoother power delivery, whereas the diesel is made by Isuzu and has an old type turbo on it which means it doesn't have a lot of power low down and then when 2500rpm comes along the car will rocket off and seem like it has much larger engine. The 1.3 Fiat Multijet / CDTi engine has a newer type turbo and when its running healthy, the power delivery is as smooth as a petrol, but with even more poke.