I spent time in both today because of this thread. These were my impressions:
F4U-4: Handles nearly identically to the other F4U's, but it's faster, climbs better, and accelerates better. With 100% fuel and a drop tank it could not sustain 3000fpm at military power on climb-out, but the performance dramatically improves as you lose weight. It's a solid gun platform. It is superlative in a dive, and can force any prop airplane in the arena to fight. For this reason, it is easy to get overconfident and put yourself in a bad position to punish a P-51 that tries to run away.
Despite its great low speed characteristics, keep it fast, and only get the flaps out in a pinch.
SpitXIV: Leaps off the runway. Climbs at least as well as the 109K-4, accelerates like an La-7, superior to the F4U-4 in these respects. The ailerons become heavy in level flight because of the high speed. It's unable to follow the Spit XVI through nose-low maneuvers that incorporate a lot of roll. The instability makes it a poor gunnery platform; the nose has a tendency to "bounce" while you're trying to aim. Requires a lot more rudder/throttle finesse to make it perform optimally than the F4U-4. It can be difficult to force the enemy to fight because they will constantly dive away in rolling maneuvers that the XIV cannot follow.
In both cases, the ability to run down the P-51D that has no intention of fighting is a major plus.