Just posted a review on the Foredom Model LX flex shaft tool I just installed on the wax carving bench. Moving the original SR version between the main shop bench and the wax bench was getting a bit old on this old man. So there was a good incentive to have the second flex shaft machine.
All accessories I own will work with either motor except the controllers. The LX is a direct current motor and the SR is alternating current. A little more detail is in the review.
I don't know if I would call the Foredom tools the ultimate quality but the quality is very high. They are the dominate tool of choice by most jewelers and any craft that requires low to medium speed rotary hand tools. The LX iwill run up to 5,000 RPM and the SR and TX will get to 15,000 RPM.
My air turbine hand piece tools, similar to what the dentists use, can spin over 100,000 RPM. There is certain types of work that need this kind of speed which is well out of the range of the Foredom products However, Foredom does have a high speed tool called the Micromotor that can get to 30,000 and 50,000 RPM depending on version. These tools are quite expensive into the over $1000 range.
Check their website if you want more details on the Micromotor.