We get this question at least once a week, usually phrased as "which one should I get" — wrong question. The right one is "what are you actually trying to do." A monocular and a spotting scope solve completely different problems, and buying the wrong one just leaves you frustrated with a perfectly good piece of glass.
Problem one: something's moving and you need to see it now
Trail walk, a raptor breaks cover, or you're sizing up a flag from the fairway — you don't have time to set up a tripod. You need something you can pull from a pocket, get on target, and refocus in two seconds. That's a monocular's whole job. Ours is the Vortex Recon R/T 15x50, and it lives in a jacket pocket, not a case.
Problem two: you know where it is, you just need to study it
Glassing a ridgeline for an hour, watching a nest from a hide, picking apart a distant flock for the one bird that doesn't match — a monocular runs out of magnification fast, and your arm gives out before your eyes do. That's a tripod job. The Vortex Viper HD 20-60x85 gets you 60x with zero shake, because the tripod's doing the holding, not your arm.
The honest answer
If you had to own one, own the monocular — it goes everywhere. The moment you find yourself wishing you could see just a bit further into something that isn't moving, that's the spotting scope conversation. Most serious birders and hunters we deal with end up owning both.