I have rode on skis with both. Used to have FKS 120s on my rossis a while back, and never ever had a problem. they were strong, never prereleased, always released correctly in a fall, and never ever broke, regardless of how hard I rode them. I really regret selling them today..
I have also rode the Jesters, which were installed on my buddy's Bridges. Its a totally different binding. It's plastic composite and not metal, so it technically is lighter by a little bit. It does not have a pivoting turntable heel like the FKS's did. It feels very similar to something like a PX 15, just it wouldnt be as strong due to the lack of metal involved. They did not prerelease on me, but then again, very few bindings do, since I weigh about 140. They do have some good characteristics about them though which I'm fond of. First off, they are set on a wider stance, which is at about 76mm rather than 67 or so that you would find on an FKS. This allows them to transfer your energy a little better to the edge of the ski while riding... that said, the FKS's hole pattern does a similar job to the ski, because it is centered right underneath your boot toes and heels. They DO cause a bit more strain on your skis though, as this puts extra torque on the core, leaving a fairly high potential for a binding pullout. That said, if you have them mounted correctly, you shouldnt have a problem.
The FKS naturally has better heel retention than just about any binding, and the turntable heel allows you to release at the tibia-fibular point, which is much safer on your knees. The jesters work in the same way that just about every other binding works, allowing your heels to push out the forward pressure and release, which can be tougher on the knees.
I swear by the FKS bindings, as well as the pivot/axial series bindings. They really are the safest, and darn near best made bindings out there. It's a true shame Rossignol/Look does not make them anymore.. and every year there is more speculation that rossignol will release that binding again.. but for the time being, that just doesnt look like it will happen...
I have written to the company, and pleaded with quicksilver, and rossi, and look... but to no avail.
I say, that if you want them to release it again, we put out some sort of petition or something.. or at least just bombard them with angry emails and stuff until they start to consider it.
The only reason I could think of why they dont make the binding anymore is because it was a little expensive to make, and they could make the binding cheaper by changing the heelpiece.. but maybe if demand is rebuilt, they could blow the world away again by making the best binding.. once again..
The skier formerly known as Rabiddingo66