I've had a 2.5 V6 for just over 18 months. Fuel economy is not bad for the size of the car and engine although I was hoping for a bit more. Maybe it's the way I drive it but I get about 300 - 340 miles to a tank which the trip equates to an average of 24mpg ish.
I've heard manual cars are quite a bit better in the ecnomy department.
I drove a 2.0 recently and was also pleasantly surprised by the power and obvioulsy the fuel economy was better This was a manual car and I think as good as the auto's are, they do sap a fair bit of power.
One thing that is a plus with a V6 is the noise - Modify your airbox, find a tunnel, enjoy!
They wouldnt have cruise control on them would they which is the muts nuts on the motorwy and decent roads. 
ok I've got a 2.5 manual with cruise. The cruise is retrofit with help of the forum. parts easily available from egay etc.
So what's the answer here. economy is governed by driving style: i get 33mpg in town by driving carefully: this compares well to other cars particularly 1.6 focus driven by boy racers who think speeding up to 40pmh is impressive (terribly sorry old boy, I am a little tired....). the 3.0 should be similar although i've never driven one.
As to top end performance, if on the odd occasion you need or want to push your cal along then you will not miss a V6: it will clear 90pmh in third whether a 2.5 or 3.0 and it will pull way beyond 30mph with ease
my very personal view is go for a V6 because of the smile it puts on your face when you open it up. Also given the green/gordon brown bridaade we are unlikely to see many V6 cars in the future:enjoy one while you can.
oh and whith the torque on a V6 it will pull in 5th from 30mph to beyond 140mph in a manual