Paula
It depends on the car really, generally you will get more mpg on a diesel but some of the more modern petrol cars are still economical. Ive had both but much prefer petrol, they are much smoother to drive and dont have that noisy start up!
My last petrol estate was a 1.8 and did just as much mpg to my 2litre diesel jeep, just have a look in the parkers guide to what your model does teh gallon, as long as its not some huge 3.5 v6 (like mine!

) you will be fine