An often overlooked advantage of caps in plunger fired inlines is the caps seal the rear of the charge better than a 209. Look at a 209 holder for most plungers and you'll see a significant space in the front of the primer. This allows moisture to get to the charge and it also increases breech fouling.
A musket cap or #11 completely covers the nipple and does a much better job of sealing off the rear of the charge. They also don't foul the breech nearly as badly.
I also found that the least powerful ignition source, consistent with good ignition, gives the best accuracy. I believe this is due to the more powerful primers cause the charge to begin to move before full ignition of the powder occurs.
I prefer Remington 11s to all others. The cap stays intact in the vast majority of shots. CCIs fragment badly, although their musket caps ( both 4 and 6 petal) are excellent.
The rifles I've tested this on are a Traditions Tracker, Knight American, Knight Mk85, 45 and 50 caliber TC Black Diamonds and a TC Omega Z5 .50 caliber using 209' s and Variflame adaptors with small rifle and small pistol primers, both regular and magnum versions.
11s work fine with black powder and loose Triple 7.
Good luck!
