Just keep in mind what kind of drivin your doin motorway or city & what kind of milage you will be doing in a year. there would be a big difference in mpg between the petrol & the diesels.
does the 08's some into the cheaper tax bracket, I would think the 07 would be outside this.
To me the 07 diesel is the best value, but you will be paying more tax on it. Id stay away from the petrol anyway, chech out the price at the pumps & it seems its only goin to keep going up.
You will get better mpg with the diesel.
Hope this is some help