Is an ungrounded GFCI outlet safer than a plumbing-grounded non-GFCI outlet?
This is not even a contest. A non-GFCI outlet will happily send 1A through hot, into a faulty or misused appliance (think: submerged toaster), from there through you into some other ground (any ground will do, it doesn't have to be that same appliance's ground), without noticing anything's amiss. You, meanwhile, are dead several times over. The ungrounded GFCI-protected outlet, OTOH, trips as soon as the current imbalance reaches 30mA or so. If all the current leaving the GFCI eventually returns, there's none left to be going through your heart.

Yes, the GFCI could fail to open when it should (so test it regularly), but the counter-objection is that having a ground on the outlet doesn't guarantee that the circuit breaker will operate when a loose wire in an appliance touches the chassis. This is why my local electrical code doesn't like the idea of a single failure causing harm, so all socket outlets must be both GFCI-protected and grounded. For some reason I don't quite understand, lighting fixtures are exempt from this, as are stoves/ovens.