Re: Spotting scope for partner or rifle scope?
No sarcasm intended, but do you really think your buddy is gonna want or need a 20X on his AR? If you are talking a variable power scope that goes up to 20X is one thing, a fixed 20X entirely another. You didn't specify which.
Off the top of my head, here are a few reasons for a spotter....
1. What if a buddy can't go? Having your own spotting scope solves that problem. Set up on a tripod beside you, shoot, then check through your spotter.
2. More often than not you'll get more magnification / higher zoom power out of a spotter. Your needs or desires dictate the zoom range required.
3. Spotting scopes generally have larger objectives, allowing more light in, thus better viewing in low light..... assuming you buy a scope with good glass to begin with.
4. Just a personal thing, but I'd rather look through a spotter for long periods than a rifle scope. YMMV
If you list the ranges you shoot and what you are trying to see you might elicit more responses. Also, what is your budget - what's the absolute most you'll spend?
Depending on what you want to do, you may get buy with just buying a higher end scope with HD glass, crank up the zoom and look. That's an option if that would work for you.
Are you just going out to a range or wide open areas, setting up in one place and shooting targets? Do you hunt and have the need to set up and scan vast distances for game? Are you willing or able to hump a tripod and spotting scope realizing that there are vast options in size and weight for tripods as well as inexpensive to stupid expensive. Same goes for spotters. You can go for smaller and more compact, up to the full size Kowa, Leica, Vortex Razor, etc. with 80 mm objectives spending anywhere from several hundred to several thousands of dollars.
A quality spotting scope is an asset and can be an expensive one at that. Buy the best you can absolutely afford. Good glass doesn't come free or necessarily cheap, and going too cheap will just cost you more down the road when you decide you have to pony up what you wasted on the first one and add some more to get something decent.
I use a Vortex Razor I picked up off of SWFA's sample list for around $1200. Do some googling and you'll find is competes amazingly close to the top end Kowa's and other "alpha glass" which can cost 2 - 4 X as much. Do I personally need a scope this good? Honestly, no I don't, but I can say without a doubt that whatever I use it for, I've never wanted for more. This is an instance where "buy once, cry once" sure came into play and I have no regrets.
If you have the opportunity, politely ask other shooters you know or see where you shoot. Most folks will let you check out their gear if you are respectful and careful. Look through as much glass as you can and let your eyes be the judge.
At any rate, good luck and happy shooting.
If you want to do some more reading, check out this link. ILya is a member here on the hide and his elvaluations are highly regarded.
http://opticsthoughts.com/index.php/spotting-scope