Mike, I really can not address the quality vs quality of the skirting and the tooling leathers, because I use only really use the skirting. The last time (about a year ago) I used any of the tooling leather I was greatly disappointed when, by comparison, it did not oil up and finish out even close to as nice as the skirting.
I use the exact same leather is now five colors ( russet, chestnut, brown, black and havana) and two split weights (7/8 and 2/3) and get very satisfactory and uniform results. It is all the skirting leather. It also seems to have more "body" than the tooling leather.
The one and only advantage I can possibly see in shoulders is that they are trimmed out a little better and are smaller square footage than a full side hence less expensive over all, but they do cost more per square foot.
Paul