We've had the MORryde IS and disc brakes installed on two fifth wheels. As full-timers who travel a lot (rather than staying in one place) I wouldn't be without it. If my Google search pulled correct information, the GVWR for your fifth wheel is 10,122 lbs. - a bit on the light side for even the 7K MORryde suspension units. If I were in your shoes, I'd have the disc brakes installed and pass on the $6,500+ for the IS.
Rob
EDIT: OK - apparently you updated your RV info while I was typing. What I said, but the decision on the IS is yours. We really like it.