Is there a difference between the FDA approved 'Flex Belt' and EMSculpt, besides cost? And if so, what is it?

There are many other at home and medical devices that stimulate muscle contraction, however these are usually weak contractions. EMSculpt is the only device in the world at this time that can stimulate the muscles to a level of "supramaximal contraction". This is a level of contraction that we can never achieve on our own working out. That's why a single treatment is the equivalent of doing 20,000 sit ups. There are several clinical studies which have shown a 16% increase in muscle mass and definition after a series of 4 treatments. There is no other device that can do this at this time, and any other treatment is a waste of money. Hope this helps answer your question.

Thank you for your question.
You are correct in that both devices stimulate muscle.  The similarities however end there.  
The Flex belt is an EMS device and offers a very localized electric muscular stimulation with about 150 contractions a session. The intensity can be adjusted from 1- 150 at each session.
The Emsculpt uses HIFEM(High Intensity Focused Electromagnetic) energy to create a magnetic field in the tissue.  It penetrates 7 cm below the skin and polarizes all the motor nerves to stimulate all of the muscles in the area very intensely.  It is an MRI grade device and is very safe.  Electrical TENS units have a risk of shock and burn that the FDA does warn about. The intensity of the contractions with Emsculpt is far more significant than the Flexbelt.  In a 30 minute session you will experience 20,000 supra-physiological muscular contractions. This generates an average increase in the muscle of about 16-19%.
Along with the muscular stimulation, Emsculpt was found to have an effect on the overlying fat layer causing cellular overload and fat cell death.  In other words, permanent fat loss of about 22% in the treated area.
Hopefully, this comparison answers you question.