@TxGirl, for some reason I couldn't "read" your post correctly before, my brain was not cooperating. Sorry if my answer is incomplete. I'll try to remedy that.
The body of the bags are similar in that they are totes with two interior pockets and unlined, but very different in how they behave and feel. The Shop Bag is just significantly less substantial. It weighs less, the fabric isn't as stiff, though ultra durable. It's like a bulletproof membrane around a skeleton/framework. The corded seams internally give the bag structure. As you have noted it's side panels with a unibody front, bottom back panel. The pockets are attached at the bottom on the sides, they have no closure, there are 2 o-rings at the top of the bag. The base of the bag is rectangular. The bag is curvy, but overall it is much more squared off than the LS. Pointier things will distort the shape of the SB, but because of that it also somewhat has a better capacity to adapt to unusually shaped items. Most items would be hard pressed to actually puncture through because of the strength of Dyneema. The SB has no top closure. Though some carry the SB as a purse, it's more truly a multipurpose tote in my book. The SB was introduced when the Utility Tote was phased out of production. The shop bag is androgynous.
The Little Swift is sturdier like a fabric bucket. The structure comes from bound seams, the seam placement and contour of the panels and the Ballistic Boot around the bottom. The fabrics (Ballistic, Cork, Hemp on Rubber backing and most recently two in Cordura) have a much sturdier feel and the ability to for the most part stand on their own vs. Dyneema. The bottom is oval shaped. The main pockets are lined, have zipper closures, o-rings and a urethane panel that also adds a stiffening quality as it's stiffer than Dyneema. These pockets are attached to the top of the bag's front and back panels. There are two additional slip pockets formed by the cross over of the side panels giving the sides more structure due to the fact that there are two full layers of bag fabric forming a triangle shape at each side that extends around the bag. There are 4 additional o-rings within the main compartment. The LS has a top button tab closure. The LS is more of a purse. This bag is almost certainly just for the ladies.
The bags are similar in their handles. They are composed of a section of binding that also encases the upper rim of the bag that transitions to a Poron foam padded portion. The fabric on the LS is Cordura as we have seen in the bags, the SB has a thinner nylon. The inset on the LS handles is Ballistic, the inset on the SB handles is Dyneema. The drop measures 10" on both. Both are extremely useful bags, but I personally do not see them as interchangeable. They both have black accents, but the LS accents stand out more because of the boot and thicker trim binding the external seams on the bag. Though the measurements are very similar, 11x10.5x6 for the SB and 6x9.5x11 for the LS, the orientation of the dimensions, the base shape, and the side depth contribute to the SB having a larger capacity.
I hope this wasn't too boring and helped.