Here is a pretty good article on Spotting scopes,
http://www.6mmbr.com/SpotterReview.html
The best way to determine what scope you'll need is to actually compare different scopes with what you are planning to look at. For example when I bought mine I wanted to see bullet holes at 500 yards, I took a target and set it up 500 yards away then we got out a few different spotters and compared. If money isn't a huge issue then just buy a Swarovski and you'll be good to go, Leica and Zeiss are very good also. The HD glass is worth the extra $$ I think. If your planning on spending a fair amount of money and the optics place you are dealing with isn't going to take the time to please you go somewhere else.