I have owned the arm that is on the left in the picture for many years. The red one on the right I bought about two years ago and was too busy to do what I wanted with it. Only just recently did I find the time to make my idea then I posted it here as an instructable.
When writing my instructable, I did a quick search online and also discovered that there was a wide variety of prices. The most obvious thing that I could see that made a price different was that the small knob that does the fine movement of the end of the arm was not on the cheaper models.
Please see the video that I added to this instructable to see how this fine adjustment works.
Given a choice, I would choose the ones with this fine adjustment.
I was also unsure if the cheaper ones where shorter in length and I will tell you that I would not want a shorter length.
It is worth noting that these arms are used by machinists to measure very small amounts. I expect that very high quality arms are needed for very fine machinist work. However, I could not see the need to that kind of quality and chose lesser expensive ones.
I also noticed that some of the more costly ones had a further fine adjustment capability. They had an adjustment just above the magnetic lock. It would appear that this adjustment would make the entire arm tilt back and forth. I can see an advantage to having this adjustment. However it was only available on the more costly ones and I am not sure that it would be valuable enough to justify the extra cost. Having said that, I have not used one with this additional adjustment, and if I did get a chance to use one of these, I may like it. Perhaps one day I will get one of these and add my thoughts about it as an edit to my instructable to let you know about my experience.
Price is a difficult way to measure quality, but I can tell you that at the time of this post,( May 2017) the ones that I use cost about $40.00 US dollars.
I hope that I have been helpful. If you have a further question, please ask.
It will be my pleasure to answer it.