It's a bit unfair to compare them... Both had different purposes and were build/designed/used accordingly. They also were of different ages. I know a few years appears to be nothing nowadays when an aircraft can easily go into 10 years of development before being accepted in service, then spend half a century being continuously updated for new type of missions, but in times of war, like WWII, development was on a higher gear and a couple of years could mean differences between something decent and something really good. Just check out what the USAAF had in its arsenal in 1941, and what it had in 1943.
The Spit would not have done long-range escort over the Reich. It was not meant for that. But it did marvellously well over England in 1940. Probably the P-51 would have done better against Me-109E, Stukas and He-111, but it was not available there and then.