Apart from them both having Aspheric lenses in the eyepiece lens assemblies they're anything but similar - the HGs are about 120g lighter on average then the BDs as they use a magnesium alloy body rather then aluminium, the quality of glass (from Schott AG) is better, the prism mirror coating is superior, the light transmission is better, the contrast is 17% more than the BDs, they've got a locking diopter, dual gear system for the focussing, much better DOF and 4 click-stop eyecups.
And all this for about £120 more on average across the range...
Hope some of this is of use to you!
ATB,
Dave.