To answer some of your questions;
When I was looking to buy my GTI I managed to get a GTI & a GT TDI for a day a piece to test drive and assess - not an easy task. My conclusions were that both cars were very easy to drive & live with. Both have very flexible engines and from the inside, if you ignore the obvious styling differences, in everyday use (i.e. not trashing along your favourite stretch of country road) the most obvious difference between the GTI & the TDI is the extra refinement of the petrol engine over the diesel. That's not to say the diesel is at all noisy, you only really notice it when you accelerate briskly.
The GTI is undoubtedly a sharper drive (largely thanks to it's steering & suspension set ups) and predictably the petrol engine enjoys being revved (& is far more rewarding when driven hard) much more than the diesel. However there is a considerable difference in their thirst for go-juice, the diesel will do 48MPGish, where as the GTI will do 32MPGish, same driver, same conditions.
In fairness neither car is better than the other in the right circumstances, it all depends on what sort of driving you're doing and how much you want to pay the extra for the GTI & it's inevitable extra fuel costs as to which you should go for.
For me it had to be the GTI because I live in a rural area & I do a lot of cross-country driving (fast, sweeping mountain roads where you can really push a car along). However if I lived in the middle of a large town & I did loads of long distance Motorway driving, I'd probably have gone for the TDI just because of the economics.
You pays your money & takes your choice.
