Difference between the 75mm in the HS129 and in the B25 is that the HS129's gun is a slightly remade anti tank ground gun, it's got a muzzle velocity of about 3300 feet per second and can penetrate about 130mm armor from 1000 meters distance (meaning it could kill every tank on the battle field easyily). On a typical pass, starting from 1000 meters away, it could fire around 3-4 rounds.
The B25H used a low velocity 75mm, short barrel slow speed of the shell, far from as effecitve against tanks as the HS129's gun. B25's guns was more a gun for softer targets like veichles and fuel dumps etc. Then again, B25 could still fly pretty well with the 75mm, the HS129 was reported to be a brick (even more so then without the gun).
I want the HS129 though
