All depends what your going to be using it for mate.
If like me its 4miles to work in stop start traffic then the whole diesel thing is pants unless you wanna drive like a gran
As like I said im getting the same MPG out of my Remapped GTi
But if your commuting a fair bit more than that involving long stretches dual carrigeways etc then you will get more MPG out of a diesel.
The way I worked it out was I get the same MPG out of my GTi going to and from work and general driving but less when it comes to long stretches but the bigger smile on face in the GTi makes up for the lack of pennies in my pocket.
Unless your doing more than 15Kish a year your better off in a petrol imo.