If money is a factor, for some it doesn't seem to be, Binoculars will give you better value. For 100yds you want 10x25 min (10x30 better) and a good make. Opticron, Nikon, Hawke and loads of others. The reason I say this is, in optics, you generally only get what you pay for, and because the glass in binoculars are a lot smaller than scopes the optical quality at a given price is better.
I use Opticron Verano BGA 10x25 (?160) which give a very good image of the target at 100yds and only weigh 322g. Sat on top of a
compound or against a
recurve limb they are plenty stable enough at this range. For Sub ?100 the Nikon Travelite EX 10x25 are very good quality for the money. At ?225 the Hawke Frontier ED 10x32 are amazing with their ED glass (Extra-low Dispersion) but really any of the manufacturers ED 10xnn will be as good as you need. Worth checking out the guarantee as Opticron often give 30 years for bins over about ?250.
Once you get over ?300 you start to get into scope prices. Less than this there is very little good stuff out there and although there are some around ?100-?200, often mentioned on this forum and I see a lot of them on the line, I have yet to see one that will even get close to binocular image quality for the same price.
If you find a spotting scope is your thing, look at the angled scopes in preference to the straight. Straight are easier to start with but angled means the scope can be lower and is easier to use on the line.
A few years back I would have said avoid scopes with an objective lens (the big one at the front) less than 60mm. However today the Nikon 50mm Field Scope is very good. But in general 60mm will get you more light. As weight is not really an issue for archery, unless you carry the scope to the line each time, 80mm scopes will get you even more light and will work well even at dusk.
The very cheap scopes (sub ?150) usually have eye pieces that cannot be removed. They are often zoom eye pieces that will zoom from about 15x - 60x. Don't get over excited. These zooms are pretty much useless over 30x and most will be dodgy even at 15x. You will not need a zoom if you buy a scope with a fixed 30x eye piece. A scoped with a fixed eye piece spec'd at 30x60mm will, in theory, enable you to identify your arrows at 100yds amongst others in the boss. However, this will depend on the quality of the glass. This is where the money comes in. You need good quality coated glass. With scopes that start around ?350+, again Opticron are a good manufacturer, you will get good quality coatings on standard glass (ie not the ED glass as you will get with the binoculars at this price) but they will do the job.
In all the options make sure you get waterproof optics. Misty mornings can be the kiss of death for non-waterproof bins and scopes.
I would not dash out and buy a scope yet. Book yourself in to a tournament and walk down the line of scopes set up at 100yds. Ask the owners if you can have a look through them. You will see scopes from ?80-?800 and you might even see a top end scope as well. Choose a few you like the image quality of and ask the manufacturer and price. It's really the best way. My scope goes with me 2 or 3 times a year. My binoculars go everyday outdoors and indoors.