When you're comparing the P-51D to an Me109, it's not enough to just compare speeds and maneuverability.
P-51 has much better cockpit visibility.
P-51 has better gun ballistics.
P-51 takes more damage.
P-51 carries a lot more ammo.
P-51 carries a lot more fuel.
P-51 can drop flaps at almost any speed.
P-51 does not compress as bad as the 109.
Maybe it's me or my joystick setup, but (to me) at similar low speeds or high angles of attack, the P-51 just gets mushy at the edge of a stall, while the 109 has a nasty and sudden wing drop, which seems to be worsened by those darn slats slapping in and out.
Don't get me wrong... I love the 109 and wish that I could spend more time getting good in it. I can understand why the P-51 is so popular... apart from the psychology of "the best plane of the Good Guys" (I mean, even 15 year olds have heard of Yeager, right?)... because flying it is almost instant gratification. And it is truly fun in that regard.