Depends on the car really. My Omega estate gets about 36mpg but our mk3 MX5 34mpg. Definitely cheaper to take the '5 in most situations, only reason I don't is to try and keep the mileage down a bit.
When it comes to motorway commutes in dull vehicles these days I just sit at 60mph and let the world drift by. A brand-new small engined diesel may be fractionally better than a petrol on a run, but I've found petrol engines to be more consistent when it comes to fuel economy. Plus you don't know just how hard future governments are going to hit diesel cars given how evil they are now apparently.
"Chav kids are like sand people. They're easily startled but they'll be back, and in greater numbers."