4" is the standard barrel for revolvers for ballistic purposes, so ammo makers should be building for that. 3" is just a little short of that mark, and I hope to get the best of both worlds; +P performance, and still carry-able.
Of course I may end up with a gun that does nothing well (hey, that's what they say about me at work).
As is so often the case in barrels as short as 3", sometimes even 4", +P ammo may give only a few fps more than the standard velocity offerings. Unless the barrel is more than 3" I'll give up 25+ fps increase just to avoid the blast.