Diesel is by far a better choice all around. Mainly because it is not restricted as to the power you can create with it. Just as an example, my wife has a VW TDI and gets almost 60 mpg on the highway and over 40 in town. I've also got a chevy 2500 HD diesel with a gross weight of 9600 lbs that with very minor mods gets close to 20 mpg on 33" tires. On a dyno it created 678 RWHP and 940 ftlb torque.
Diesel will soon be bigger than ever in the US with Cadillac, Ford (f-150), chevy (1500), suburu, kia...etc all making small block diesels available as early as 2010.