I've had both the 3.0l with the 5 spd (2001), and the 4.4l with the 6 spd (2004). Gas mileage is essentially equal between the two. Average is around 18 mpg. The combination of valvetronic and the 6spd or the '04+ cars really enabled the V8 to basically equal the six cyl. The 3.0l is a great motor and it feels very willing (even in the X5) due to fairly aggressive tranny mapping. If you are satisfied with the 3.0l performance I would probably go with that due to the fact it is easier to maintain. If you think you might be able to use the extra power - towing or with heavy loads than go with the V8.