To start with, go here : www.honestjohn.co.uk/realmpg/

Now, it's not perfect - nothing can be - but it's what people who use this site have put down as the mpg that they get. Some will be calculating it out, some will be guesstimating, but the idea is that it all evens out, and you get an average of an average.

According to that, the Yaris 1.3 VVTi is averaging 45.3 mpg. The Fiesta ecoboost is averaging 46.0 mpg

As to what engine type to recommend, it all depends on your TYPE of driving. If you are doing lots of urban/short runs from cold, then modern diesels are far more likely to give you problems in the medium/long term, and a petrol engine would be more suitable. If, on the other hand, you're doing long runs (as in 30 miles+), then diesels might work out better. 10,000 miles a year is still within the mileage where petrol generally works out cheaper over time.

If the Yaris is problem-free, then I'd be inclined to say to stick with it. You won't get much (if any) better real-world fuel economy from a newer petrol car, unless you buy something horribly gutless, and drive it like a funeral director.