I read a long time ago that the CCI Magnum cap was actually hotter than a musket cap, but don't recall where I saw it or what proof was offered, if any.
I used musket caps for a while on my Greyhawk, but had trouble with FTFs. After trying to figure it out for a while, I called TC and the tech told me to check the length of the nipple. Sure enough, it was too long, which changed the angle the hammer struck at and caused the FTFs. I never got around to buying another musket nipple, but just went back to #11s.
The main advantage of the musket caps is that they are much easier to handle. Thy do seem to be a bit scarce these days and all caps are pretty expensive; much higher than regular 209s bought by the 1000.
