The 51b is in its element up above 14k. Below that, the 51d is the better performer. I prefer the D in almost all cases due to the better firepower and better low-alt performance.
Flaps usage is a tricky one. Basically, I don't use em until i have to. If your flying the 51 to its strengths, eg keeping it fast, then using flaps wont increase your turn rate very much, they add drag which can be useful at times, but it will slow you faster. The pony isn't too good at winning slow-speed turn contests. Keep it near its best corner velocity (270-300 ish) and you can turn with most aircraft in the game if you're smart about how you turn.
If you're over 300 then flaps will give you no advantage, IMO, as you will still be able to pull max Gs without them. Between 270-290ish a notch of flaps can help you maintain max Gs. Get too far under that, and you're really committed to fight and due to the slow acceleration qualities, you may not be able to escape if things go wrong. In this situation i get 2, 3 or even sometimes 4 notches out at my slowest points. But remember if you don't need em or want to accelerate at your best rate, get them up!! Bring em up as you go into the nose-low phases of the fight etc. When you're clean, you accelerate (build E) faster - which is critical. Its a matter of deciding which is best (accelerating, or turning tighter) at any given moment. You should really be aiming to kill by way of snap-shot (rolling scissors, overshoots etc), not trying to turn harder to get into a dead-six position. Six position is good, but don't waste all your E trying to muscle your way there.
But remember the pony's strengths are in its excellent high-speed performance and good instantaneous turn rate, not in a slow-speed stall fight! Flaps wont turn your pony into a spit, so don't expect them to. Get em out if you need em at a specific point in time for a specific reason, but get em back in pronto when you don't really need em.
What times are you normally online? I'd be happy to help, the P51 is my main ride of choice. But i fly weird hours.