Which are more reliable, sensors with current varying output or voltage varying output?
If you mean industrial type sensors then typical configuration options are 0 - 10 V or 4 - 20 mA.

An article on SensorZone says that for 4-20mA systems:

Advantages


It can be run over long distances with minimal signal losses compared to voltage type signals
A varying current loop load impedance or supply voltage will not significantly affect the signal as long as it does not exceed recommended component limits
Rugged signal with low electromagnetic susceptibility
Saves on cable wire becasue it only needs 2 wires to function
Live zero reading verifies sensor is electrically functional


Disadvantages


High power consumption compared to other analogue signal types
Elevated output at zero reading
Supply not isolated from output
Increasing circuit load resistance, will reduce the supply voltage available to power the transmitter that is generating the 4-20mA signal.