When I had my Mk6 TDi Bluemotion I used to challenge myself to beat 100 by adding together the average speed and average mpg for a journey. On one trip from Southampton to Hull and back (on the same day) I managed to average 50mph and 70mpg. I started out with a full tank, and when I got home I had 100 miles worth of fuel left, according to the display.
No chance of that with the GTI, and I would need to stop for fuel on the return journey, but I don't cover as many miles now, and I prefer the GTI for everyday driving, as it's less compromised than the diesel, and just more fun to drive - for three years when I had the Mk6 it shared my parking area with a Honda S2000, which had Ohlins suspension and a few other tweaks. Somebody made me an offer I couldn't refuse for it, and with the fallout from VW's spectacular own goal regarding the emissions scandal, diesels weren't likely to remain a viable option for private ownership from a depreciation viewpoint, so I took the opportunity to buy one car for all my needs/wants, which meant a fast Golf.
In everyday use I can achieve somewhere around 37 - 42 mpg, helped by being surrounded by 40mph speed limits. I keep flirting with the idea of a Golf R, but my practical, pragmatic side counters these moments of whimsy with the cold hard reality that an Up! would suit my needs better. We buy the versions of otherwise sensible cars which have been moved further up the food chain by the addition of more powerful engines, harder suspension, better gearboxes and so on largely for reasons of ego and/or vanity, and/or the occasional adrenaline rush although as all cars are compromised, the faster versions are more compromised than their less exciting stablemates, yet are more desirable because of their ability to excite.
I'll continue to enjoy the compromise that is the Golf GTI. It is a good trade-off between performance, practicality, and economy. Everything is a compromise, it's just a question of finding which compromise works for you.