Verdict : Workfit-S by a longshot (mostly).
There is certainly nothing wrong with the Workfit-A, but it boils down to just being extra fickle for a level of motion and configuration that I don’t actually need. It wobbles a little and, while entirely manageable, it is not something I really wanted to consciously manage. The entire product is of phenomenal quality and it is excellent in its own regard, it’s just only extremely useful for people who really need the range of motion that the arm provides. If you are just looking for a convertible standing desk, then this is not it. If you are looking for a high-quality standing option with an extremely wide range of motion then this might be worthwhile. I could see this being useful in open-workplace environments where you might need to show a client or coworker your screen very easily.
The Workfit-S, on the other hand, is exactly what I wanted in a standing desk. It is secure, sturdy, but still very portable so that you can feel comfortable moving the hardware around easily if you need to. The distance between the keyboard tray and the maximum monitor height also feels a bit greater, though I don’t have the actual distance. I ended up wedging the included T-shaped allen wrench underneath the keyboard tray so the keyboard is at a fairly extreme incline. This allows me to have an amazingly comfortable setup that actually eliminated my RSI pain within one week. The mouse ended up being used on the desk portion but, with the keyboard being lower with my alternate config, the height for everything still works very well.
Even without configuring the keyboard tray against instructions, it is still an excellent product. Unfortunately, for me, I committed to the Workfit-A at home and Workfit-S at work, so the one I prefer less actually came out of my own pocket.
The Workfit-A is still great and I am getting use out of it. It really needs a secure base so a cheap desk is only going to make the wobble worse. I ended up securing mine to the wall along with the desk and that did help a bit, but it’s also in the corner now so any benefit from the articulating arm is pretty much gone.
Overall, the experience with Ergotron products was incredible. I was happy with both purchases as soon as I opened the boxes. The build quality is really, truly excellent and everything goes together very smoothly. They both cost a lot ($499 each) but it is entirely worth it. You won’t feel the least bit ripped off.