I think that's really hard to come up with anecdotal evidence if grapefruits are hardier than an orange. The reason of this is that while the relative hardiness may be in doubt, i think all of us agree that acceptable grapefruit need more heat than oranges; now, the places that in winter are colder, usually have also less total heat, so grapefruits aren't exactly a desirable plant there, and most probably got replaced, so in any given time there are few plants of grapefruit that are grown in paces where oranges can't.
The difference in anecdotal evidences may also be related to physiological factors that have nothing to do with hardiness. Grapefruit in general are more vigorous so probably spend more time than oranges growing. Even if hardiness while dormant would be superior, they may be anecdotically reported has more tender because is more probable to see one of them with frost damage due the longer time the plant spends growing? I'm doing a wild guess here, nut i also guess that a good rootstock may play an important role here.
Reading around however I got the impression the grapefruit can do better than orange in cold department.
However, in my city with temps going every several years and then around 14F, citrus are relatively rare (with the notable exception of the bitter orange). Oranges don't grow here, few plants have started to happear in last 4-5 years which have been exceptionally warm.
Recently I have found this plant that reminds me of a grapefruit. Seems seed grown, I haven't been able to inspect it, except from the road.
This isn't a 5 years old plant, and in 2011 we went to 12F.
The sometimes you get the unexplainable. I have a Tahiti lime that seems to have survived 24F; a small cutting, own roots, good place but nothing exceptional. Will keep an eye on it.