Ultrasonic Fqis
Digital capacitance and ultrasonic are equally accurate. It's the software that provides the accuracy and that it is constantly evolving.
As far as maintenance goes there is a fundamental difference which the industry hasn't managed to grasp.
In capacitive system it is the in-tank wiring looms that degrade. They really need replacing after 5-8 years as their capacitive properties change. The probes themselves are just tubes of aluminium, so if kept clean rarely fail. Where a probe is replaced it is likely to be the diode block, or the connector. But maintenance crews normally change the probe as a way of avoiding getting in the tank to repair a connector or change a loom.
In an ultrasonic system, the looms are not capacitive and shouldn't degrade. But the probes are the active devices and do fail. If a probe fails it is removed from the system by software, and each cell of probes can tolerate 3 probe-failures before you need to enter the tank. It is therefore the fact that harnesses changes are less frequent that should be the advantage of ultrasonics.