had a similar dilemma earlier this year
We ended up buying a very high spec Mini Cooper D.
we looked at the 500 and decided that it was too small felt small and lacked in luxury and quality feel.
Cheaper Minis such as the mini one and the lower spec coopers feel very similar to the 500. we went for the diesel purely because of the huge number of miles we do (no fiat 500s have cruise control), and here in England the road tax is free on the diesel models.
this worked out to about 21,500 euros so much higher than the costs you're looking at.
if you are doing lower mileage then there is no real need for the diesel, would probably cause more hassle than its worth, personally because of your budget and needs I would suggest you go for a fiat 500 both hold their value fairly well but if you're spending the sort of money mentioned then you'll see no real difference in the features between the two cars.
a lot of the comments above seem to be based on the older mini which is a very different car to the current post 2007 model (even if it does look the same)
i'm 6'1" and found the 500 to be a little cramped, you sit very close to the passenger and i had the seat back as far as it would go.
the mini has a much 'sportier' seating position which far more adjustment of the seats, you seem to sit lower and if you put the seat right back i can't touch the pedals, this obviously comes at the expense of rear leg room (actually no rear legroom at all with the seats back) width wise the mini gives more spaces between the doors which gives the car a much bigger feel. where as the 500 feels like a small car
both are great fun to drive in different ways, the 500 isn't fast but handles well and loves to be chucked about. the mini is faster with a much stiffer suspension drives like a go cart and doesn't roll much in the corners, the disadvantage being a broken spine on bad road surfaces and speed bumps, but is not as tiring on long motorway journeys
boot space in hindsight is probably not really any different however the mini has a little trick to give a bit more boot space and still keep 4 seats if needed.
obviously the choice is yours however i think you'd find the 500 a better choice for you and that's coming from someone who bought the mini i like both cars and would happily have a 500 to replace my punto however its still very small for our needs feels small and probably wouldn't be so happy to eat the miles. mini also have something called TLC which means we don't have to pay for servicing, in fact our only expense for the first 3 years are tires and fuel everything else is covered.