It all depends on your skill level.
The 109K hast more stellar performance as a fighter when you have an experienced pilot at the stick. However, the P-51 is much more 'accessible' to players with lesser skill level, it's strengths don't need so much training to be played upon (grammar?)
As I said earlier, the F4U and 109k have enormous potential once you have mastered them. But they are more difficult to master than some other fighters.
I remember when I went from La7-Ki84 to the 109K and F4U, after being so thoroughly being thrashed by them on several occasions.
It was an instant disaster, as both are so peculiar compared to what I have flown before. Much more than with the other planes I had to acquaint myself with those two on an individual base.
Just having a huge potential doesn't equal 'easy mode'