The P-51B is a significantly better handling aircraft than the P-51D, in my opinion.
It turns better, it is more stable at low speed. The D-stang has a much nicer view though.
As far as a B stang vs a D stang in a fight, I remember a while ago me and Wldthing used to fight in P-51s quite a bit. If it was D vs D, he'd own me probably 75% of the time. If I was in a B, and he was in a D, I'd win about 75% of the time. It makes a fairly big difference in a dogfight.
Of course, niether model is a Spitfire, and they don't really have the insta-kill firepower you'd need to fight the way I do... so I don't fly them.