Sounds like your mind is already made up!!! If you've always wanted a hot hatch, and loved your MK3 GTI, then you will simply have no regrets and disappointments with the MK5.
At the end of the day, if you can afford to run the petrol motor of the GTI then I see no reason to not buy one!
Although I can't advise you on the diesel, I will do my best for the GTI:
MPG - I think a real life example is good for you. Earlier this year I was driving 50miles to get to university every week. It took 1 hour to do the journey. 30mins spent on 30-50mph townish roads, the other 30 mins spent up the M25, cruising at 70. Each week, I was getting 42mpg without fail. No traffic.
I have since changed universities, and my route is a 17.2 mile journey which is mainly 30-40mph town driving with traffic here and there. Each day, I don't get below 32mpg.
Bear in mind that the above examples are just driving sensibly and striving to get good mpg.
I will make no bones about it - when I am trying, the mpg WILL go down to 22-24mpg. When driving as hard as possible, I haven't seen it go below 20.3mpg. But you really have to thrash it to get that low!!
Range: With a mixture of driving styles, I see no reason for you to get less than 280 miles to a tank. A full tank of driving sensibly will get you 350miles quite easily. Lol, I'm sure someone will come along in a minute and say 'hang on, I can't get anywhere near those figures!' Every car is different, and everyone's driving styles are different!
I think if you get the diesel, you will always be thinking "hmm I really wanted that GTI" !
Hope this helps you a bit
