Both very good cars for the sector they are in and both will be reliable. 40 miles a day is about 10,000 miles a year, so with private mileage, you may prefer diesel to petrol for economy and in that area the Ford is likely to be better and will also be a better 'drivers' car. The Fiesta will put a smile on your face if the commute is over twisty roads. If it is along plain main roads and traffic/motorways, then either will be fine.

Approved used cars are better than non-approved. Hopefully they have had a thorough check over. Don't waste your money on an after market warranty. Simply put money to one side instead.

Also look at the Skoda Fabia and Kia Rio.

I would go petrol just to eliminate the possible diesel related problems, and you will get mid 40s economy from most small petrol cars over that distance. I would probably go Japanese for a second hand car, and look at a Honda Jazz, Mazda 2 or Toyota Yaris. All are very reliable.

A further possibility art the top of your budget would be a brand new car such as Peugeot 107, Citroen C1 or Toytoa Aygo (All 3 are really the same). Dont be put off by the small external size, and you might better 50 mpg with one of these, as well as having at least 3 years manufacturers warrenty. The drive pretty well too.

Both my sister/brother-in-law and ourselves bought new i20's in April 2012. So far they haven't put a foot wrong. As regards comfort, my sister and brother in law range from 5ft 3 to 6ft 4 and have found the i20 comfortable enough to undertake holidays travelling from the UK to Germany with a full load of luggage and two bikes on roof mounted carriers. We have done around 9500 miles in ours, and have no complaints with either the comfort or the reliability. The comfort of the seats have been absolutely fine

Yes they're not the most exciting to drive, but performance wise they're certainly on a par with our previous Mazda 2, and the ride, through not having low profile tyres, is far better. Ours are also well equipped - six airbags, bluetooth, etc.