What is the difference between a GFCI ungrounded circuit and a grounded circuit?

I think it really depends on the value of your grounding electrode, your grounding electrode system connects to earth and provides a path for ground faults and this path is supposed to assist in the fault clearing or tripping the breaker.

The problem is what happens if your body is in the fault path the breaker won’t trip until you are long gone, introduce the GFCI they measure the hot and neutral when there is an imbalance or a fault above 5 ma they interrupt the current so you will still be here having just received a little jolt Or shock.

A grounded system is best as it takes the protection of the ground to each device. the GFCI circuitry only uses hot and neutral so the ground is an additional protection With some metallic hand tools the ground prevents you from getting in the path of a fault.

Protection, in theory, no. The GFCI takes care of the “killing” risk of an outlet, at least, it does as well as grounding does.

However actual grounding does something GFCI does not: provides a place for natural electricity to go, notably ESD from shuffling your feet across the carpet in winter. With a ground wire, that simply goes into the building’s grounding electrode system.

Without a ground wire, it finds a place the equipment is not insulated for 20,000 volts (many such places), and leaps to the hot or neutral wire, then goes through equipment if necessary to the neutral wire, then via the house’s neutral-ground equipotential bond to grounding electrode. The equipment might not enjoy this too much.