I voted for the 155 mainly due to the gun I'd be firing it out of, a P-40. Short barrel, light gun. These are my reasons:
the heavier round (with the same charge weight) will already be going slower than the lighter one (lighter is faster), then add to that a shorter barrel which means less time to accelerate this bullet, which means even slower speeds. HP's need speed to open up, and most HP designs were not created around a short barrel, which means they were probably optimized for a longer one, and with the speed loss from a short barrel, may not consistently expand. Also adding more of a charge is not always an answer, as much of it will burn outside the barrel adding muzzle flash (not good) and not speeding up the round all that much.
Then factor in the fact that lighter rounds recoil less, which is very important for small light pistols getting accurate follow up shots, and that they may expand more reliably out of a short pistol.
Also lighter rounds tend to be shorter and sometimes feed better.
Bottom line though is to make sure what ever you choose functions well in YOUR pistol, and remember to take barrel length into account when looking at any performance data for ammo, or just use yours to do your own informal testing. If a lighter round performs similarly to the heavier one, why subject yourself to the greater recoil and slower follow up shots for the same result? If the heaver one performs better, then you have to weigh in all the factors and see what is most important to you.
