I guess the obvious question is, what size soft bag are you looking for, as they vary wildly. Anything from a very small bag, just about large enough to put your sandwiches in, through to very large bags, capacious enough for two weeks’ camping, all your clothes and more besides.
Such is the spread of sizes, I have two or more soft bags. The two bags shown in the opening post, plus a very much smaller bag and something between the lot. All of them I bought secondhand and were, on receipt, in very good condition.
Ease of fitment? Most, if not all, soft bags will (sometimes with a bit of invention and imagination) fit well. It sometimes takes longer to take them off, than put them on. The latter is maybe a point to consider, if you intend to take the bag off each time you stop. The BMW bags have clips on the retaining straps, which make removal easy. Similarly, the BMW straps are easy to tension, holding the bag in place. You also get various strap lengths, making the voice of fitment position easier. For bags which are less easy to remove, I use an inner bag, which I simply lift out, leaving the soft bag in place. That the inner bag is waterproof (not all soft bags are) is an obvious advantage, too.
Anything else? I think a shoulder strap is a good idea. Why? When stopped, for instance when going into a hotel. you inevitably end up carrying stuff and your hands are full. Being able to throw a bag over your should and leave at least one hand free is a clear advantage.
