You will get higher quality optics for the amount of money you spend with a spotting scope, since essentially a pair of binoculars = two spotting scopes mounted together. You will also have lighter weight for a spotting scope vs. comparable binoculars, for the same reason, which is important if you're going to be carrying them around any great distance.
You can spend more time looking through binoculars without eyestrain than a comparable quality spotting scope, and your field of vision will be somewhat larger.
For 10X and under, binoculars are manageable and most people would prefer them. For 20X and over, binoculars would be too cumbersome and you would need a spotting scope. The crossover seems to be at 15X. There are some wonderful 15X binoculars, but they are a bit too heavy for my taste, and if I'm going over 10X, I'd just as soon jump up to 20X with a lightweight tripod which will really enable me to see things I can't see with my normal binoculars, which are 8X.