Many good points have been made for the reasons each of us chooses a certain length of muzzleloader. But I think Daryl has hit on a key point...balance point and the overall "feel" of a gun is, in many ways, more important that if the barrel is a few inches longer or shorter.
As an example, I have two .54 caliber 44" swamped early Virginia styled rifles. One was built by the late Larry Williams and one by Mike Brooks. Hands down, the Larry Williams gun, which is a little over a pound lighter, has as perfect a balance as I think is possible, and is the gun I'd rather carry around. The Brooks gun is built around a very highly figured and very dense stock wood which adds overall weight and also pushes the weight back a tad more than the Williams gun. Both are excellent shooters. While I'd rather "look" at the Brooks gun or shoot it at the range, when it comes to hunting, I'd rather carry the Williams gun due to the overall weight and the weight distribution.
I also have a Haines copy with a .54 cal 38" swamped barrel which is a little lighter than the Williams. While the balance is nice, the Williams still has something about it that one just has to feel. Everyone who picks these guns up and points them makes the same comment....the Williams gun is 100% the choice they would make just based on how the gun feels.