I have used both. TanktechsRx successfully cleared large blocks of struvite floating in the tank. Those floating blocks prevented complete pumpouts. TanktechsRx also restored functionality of the tank level gauge.
As for the difference - the TanktechsRx product is solely probiotic. According to the manufacturer, it is a specific bacterial recipe designed for holding tank maintenance. There are no enzymes or other materials. The action of the bacteria liquifies solid waste and also prevents the generation of the gas which forms the struvite (the grey concrete -like material that blocks the tank and hoses and interferes with the tank level gauge components in the tank.
It is my understanding that Noflex consists of enzymes, bacteria and perhaps other material. The TanktechsRx is working well for me - it cleared the initial problem (the NoFlex did not) and pumpouts are no longer an anxiety inducing problem.