One is the system that the military preferred after the debacle with a citizen’s army in Vietnam, the first time an imperial power used a citizen’s army to fight a colonial war: what’s called a “volunteer army,” which in effect amounts mostly to a mercenary army of the disadvantaged, from which the privileged are exempt. That’s why recruiters avoid prep schools and elite universities (except for officer and specialist training), and work hard in poor neighborhoods. That’s a “shade of grey” in between your two extremes. And there are plenty of others. Under the empire, the British commonly used sepoys — poor Indians — to fight their dirty wars, in India and elsewhere. That’s also in between. And there are a lot of other choices…
One preliminary question is whether it is a democratically determined community decision that an army is necessary. Sometimes the answer is pretty clearly Yes: in World War II, for example. There were some people who refused conscription, dedicated pacifists mostly: courageous and honorable, but doesn’t bear on the issue. Suppose that assumption holds. Then conscription is not a violation of basic human rights any more than parcelling out other unpleasant work equitably is. Say garbage collection. In a decent society it shouldn’t be “volunteer” in the sense that it’s undertaken only by people who are driven to it by need. Rather, it should be equitably distributed — which one can call “conscription” if one likes. These are basic issues discussed in all thinking about decent participatory societies, within the PARECON discussions, for example.
Suppose we make different assumptions: conscription for aggressive war, for example. Then it’s a violation of human rights because the very assumption is.
Make different assumptions and there are different answers.