STTs are also softer rubber than STs, IIRC, which also accounts for the shorter life. We've got, hmm, 15-20,000 on the 235/85R16 STs on the ECLB, 99% of it on pavement, and they look like they still have plenty left on 'em. Smooth/quiet enough (not car-smooth, but not whining) at highway speed, and we regularly get 19-20 MPG mixed driving.
I'd say STTs are overkill for your level of off-roading; STs will do fine and will last longer and ride better on-road. Wider tire will hurt fuel economy, but should last longer (generally). 285/75 and 235/85 have almost exactly the same overall diameter, so speedo calibration is a non-issue.