For a performance petrol engine they're actually pretty good on fuel, if driven sensibly, the challenge is working out whether or not you're capable of driving it sensibly or whether you'll end up mashing the limiter every time a chance arises. I know which one i ended up doing, as a result i struggled to see 25mpg on the commute lol. They will feel flat as a pancake low down compared to the HDi, or any diesel for that matter, but again, for a performance petrol engine of the era they're pretty good.
Reliability is a bit of a red herring as it depends entirely on the car you buy and how well it's looked after, but i will say this: If you're wanting something to thrash again and again and like a bit of limiter action then go with the GTi6. I'm a huge fan of tuned HDis, but the fact is the moment you start tuning and thrashing them regularly the engine suffers and it gets expensive. Naturally tuning highlights weaknesses already present and accounting for these weaknesses rapidly becomes expensive.
Reliability is a bit of a red herring as it depends entirely on the car you buy and how well it's looked after, but i will say this: If you're wanting something to thrash again and again and like a bit of limiter action then go with the GTi6. I'm a huge fan of tuned HDis, but the fact is the moment you start tuning and thrashing them regularly the engine suffers and it gets expensive. Naturally tuning highlights weaknesses already present and accounting for these weaknesses rapidly becomes expensive.