Go with a GFCI. They can/will save your life.
A probe is simply completing an electrical circuit. You can still get shocked with a grounding probe. Not so with a properly functioning GFCI.
The probes are supposed to remove "stray voltage" from the system. But what doesn't make sense to me, is that if stray voltage has no where to go, it can't pass current. However, once your complete the circuit, you system can now pass current and everything in your tank has the possibility of getting zapped from the amps now being drawn.
Its kinda like the bird on the power line. Even though the bird is sitting on a 100,000 volt power line there is no circuit to ground. So even with the potential (voltage) of 100,000 volts the bird is fine. However, if that same bird should touch the pole and complete the circuit, it is instantly fried...