I have the V6 (22,000 miles and counting), and lots miles in V8 LR3's also. The V8 is way more peppy, specially in the 20-50 mph zone, but on average, the V6 is pretty good. It is certainly faster than all the older Rovers, but honestly I don't think it's any more fuel efficient. In a long off roading trip, I'm usually a bit lower in the tank than my buddies with beefed up Disco's. My tank is 22 gallons, no idea how big theirs is, so that could be the difference.
With the V8 LR3, I averaged about 13.8MPG, with the V6, I get about 15 or 16. Nothing stellar.
Most notable thing to me is that the truck feels a bit lighter with the V8 (more nimble due to the extra power), but that the transmission is better suited with the V6. I have driven several different V8 models, and all of them were difficult to get off the line without a little bit of head snap. the throttle just isn't as well modulated at starting speeds. The V6 is much better mated to the tranny, and it's response time is immediate. It's the same transmission to the best of my knowledge.
I think you will like either one, but be sure to drive the V8 before you buy.