have you ever done the actual sums? I mean if money is too tight to mention and you do a lot of miles well then yes you'll make a saving that is significant but if you do less than average miles and you enjoy your driving, seriously i'd stick with what you've got.
10,000 miles @ 30mpg = £2,197 of V power
10,000 miles @ 40mpg = £1,648 of V power
so £549 difference over the course of a year which is an extra £10.55 a week for driving with a smile on your face or alternatively drink 3 less pints a week or 2 if you live in London and you can claim you're getting 40mpg 
fair enough, if you're seriously talking going derv then the saving will be greater but again personally speaking there'd be less chance of there being a grin on my face for not that much of a saving. I think mpg figures are thrown around quite a bit these days without much consideration of what they mean financially to the individual, especially if you enjoy driving which i figure folks must do if they buy a GTI in the first place.
I guess the fact i don't smoke and don't drink that much means i'm happy to let my car pi55 my money up the wall 
I gotta say here, I don't know where you're getting this 40 mpg from? If it's a GTI I've never seen one get a consistent 40 mpg.
And if its a GT TDi then I've still not seen one that can consistently get as low as 40 mpg.
Here's how I worked out my reasoning for having a GT TDi:
Petrol (Shell Normal): 139.9p/litre or £6.36/gallon. I do circa 15000 miles/year so would need 500 gallons at 30 mpg (average). This adds up to £3179.93 per year spent on fuel.
Diesel (Shell Normal): 145.9p/litre or £6.63/gallon. I do circa 15000 miles/year so would need 312.5 gallons at 48 mpg (average). This adds up to £2072.69 per year spent on fuel.
My car is the 2.0 GT TDI 140 mapped to 190 bhp. I regulary put the foot down, use on small B roads all week (to work) sitting in stop start traffic, and use a 50/50 split of motorway/non-motorway at the weekend. I occasionally go long trips away and wouldn't dream of changing to a GTI (cost-wise).
Howerver, there's always two sides to a story. The GTI has the looks, the noise and better handling than any diesel model of Golf so if this is more important to you then a GTI is the right car. The GTI, or Ed30 is also a lot more tune-able as well and you can easily see big power from a few mods. I get the feeling the OP posted to find out which is more cost effective.
RE the Bluemotion. I've never driven or even sat in one so I can't comment on that. However, if you are used to the GTI's power then you will miss it greatly moving to a 105 bhp diesel.
