I had a TD6 Vogue for about 75K miles and drove a 4.4V8 for a couple of days test drive before buying TD6. From this experience..
Consumption- TD6 will be about 6mpg better ( so for every 10,000 miles a year the V8 is going to cost you about £180 a year extra in fuel)
Reliability - V8's were more relaible. I had injector issues, Driveline issues and you get idle vibration issues if fit 19" or bigger wheels. May have been fixed with later TD6's though.
Ease and enjoyment of driving - V8 is better. Easier to driver, better acceleration for overtaking, quieter and the ZF auto is better than GM gearbox in TD6.
New the TD6 was £10K less, had better depreciation and had better mpg, so I went for a TD6. Buying in your position, I'd go for a V8.
Steve