I personally have not bought one, however, Vortex has come a long ways very fast for an optics company. I have yet to put a lens of theres to my eye that wasnt just outstanding.
HD glass is just a coating that really helps clarify images. When it comes to optics, any advantage you can have, along with reliablity are the deciding factors.
Once your sure you are looking at what you want, then you have to convince the wife why you need it! lol
Actually, the term HD doesn't refer to a lens coating at all. It refers to the actual material content of the objective lens. In spotting scopes, HD usually refers to the use of flourite in the objective lens material. The use of flourite in the objective glass minimizes chromatic aberration, which is the refracting of light passing through the lens. Chromatic Aberration (CA) is usually noticed as color "fringing". This fringing reduces contrast and apparent resolution. Kowa's 88mm objective Prominar spotter actually uses pure flourite crystal in the objective lens.
The benefits of HD glass helps the overall image, but is most important when digiscoping, as the camera will pick up the color fringing easily. As far as a $100 difference, the HD is definitely worth it, IMO. The image will always appear sharper with less CA, when comparing otherwise similar instruments.
Also, HD (high definition/ high density), ED (extra-low dispersion), FL (flourite), and APO (apochromatic) are usually referring to the same thing when speaking about sport optics.
The Vortex Viper HD line is supposed to be one of the best spotters coming out of China right now, so it could be worth a close look. Vortex is also a top notch company in every respect, especially customer service!