Any clue what's going on here? I understand a barrel working better with different bullet profiles and weights (surface area) but not so much powder except for extreme cases like snub noses.
You somewhat answered your own question here.
The same is true with powders and burn rates as with bullets and profiles. In one load your using a pretty fast powder and the other somewhat a medium powder or slow depending on the case. In your case it would be slow as compared to say a .357 where you could use more of it, but you could also move to 296 which is very slow.
The burn rate will effect the function of your pistol as well as the pressure developed when the primer lights the charge. The load of #5 has deterrants in it to slow it down where the BE is more like flash powder by comparrison. It hits it's peak pressure almost instantly and while it might not seem like it is slower the #5 will hit it a little after resulting in a seemingly milder start and recoil. Even if your hand doesn't notice it it does happen.
If your shooting lead bullets the BE will usually expand the base of the bullet a bit quicker which allows it to seal off and fit the chamber as it enters. THe #5 will also but it might be after the bullet has started to jam into the lands before it happens. Depending on the lead in each barrel one might have a steeper cut in the rifling than the other which may explain why one shoots each powder differently. If they both were reamed wioth the same reamer to the same throat and leads then they would shoot a lot closer than what your seeing.
As mentioned, this is why what shoots great in my pistol or rifle, may or may not shoot worth a hoot in yours. I have a couple of rifle s that will not shoot the more reccomended powders for that caliber worth a darned, but with something close in burn rate ie, RL22 instead of H4831, they shoot 1" or less out past 200yds. You might also find that your crimp may or may not have something to do with this as well. If you have good case tension, all you should need is enough crimp to remove the bell on the mouth, if your going more then you may be effecting the initial pressure as well as possibly distorting the bullet if it is cast.