Comparison of the two...
P51D:
faster at lower alts.
6 guns (makes a huge difference to people who cant shoot well)
bubble canopy
HVARs and 1k lbers
P51B
tighter turning
more stable
faster at high alts
4 guns with less ammo than the 4 gun P51D
annoying older rockets and 500 lbers max.
In my opinion the P51D is 5 due to the 6 guns, more ammo, better ord package, and the availability of speed below 15k feet. The eny on the P51B is 20 due to the ord, 4 guns, less ammo, and like said, the 51B cannot out run a D below 15K although many pilots do not realize that the B can out turn them etc.
Both have advantages and both can be flown very deadly