Having been lucky enough to see a spit and P51 side by side a year ago the differences are radical. The Spitfire looks really old against the p51 (although it looks much nicer) and it is much smaller. Designed a good few years apart and for different roles perhaps its not surprising that the p51 as has been pointed out carried twice the fuel of the spit and with nominally similar engines its going to fly at least twice as far.
Spitfires designed for point defence had to claw they way to altitude asap and then get into combat would use lot more fuel than p51 going straight and level at slow speeds for hours on end.
Differences apart it is always wonderful to see and here these iconic aircraft, the elegant 1930 sports car like spitfire and the mighty p51 much bigger-Cadillac of the Sky.
For another interesting comparison of UK and US design practices see if you can get a Tiger Moth and Boeing Stearman on the same ramp and that really does show off the different attitudes