G'day Jeff, yes, you're right, they are a bit different. Southern bluegum (e.globulus) is a bit denser at 900kg/m vs Sydney bluegum (e.saligna) 850kg/m. The big difference is in the hardness when dry as they're similar-ish when green. Certainly extremely noticeable when I hit green wood in this tree compared to the bone-dry branches and upper trunk. There are a few eucalypts that are harder when dry, but e.globulus is up there and certainly the hardest in my area. Candlebarks grow next to the bluegums here and while they're both big eucalypts, e.globulus is twice as hard when dry.
For anyone interested, here are some stats for a few species:
Species, Green hardness (kN), Dry hardness (kN)
Southern bluegum, 7.3, 12
Sydney bluegum, 6.4, 9
River redgum (e.camaldulensis), 7.7, 10
Red ironbark (e.sideroxylon), 11, 13
Alpine ash (e.delegatensis), 4, 4.9
Candlebark (e.rubida), 5, 5.9
Sugar maple, 5.2, 7.3
White oak, 4.7, 6.0