On the difference between eternal sleep and death

"... by a sleep to say we end

The heartache and the thousand natural shocks

That flesh is heir to: 'tis a consummation

Devoutly to be wished - to die: to sleep - " 

From Hamlet's soliloquy, "To be or not to be", Act 3, Scene 1, Hamlet(Second Quarto)

I watched "Star Trek; Into Darkness" the other day, and one of the concepts intrigued me. Bear with me if you haven't seen it; this post may make a little more sense if you have, but not much.
Khan and his crewmates are sent to sleep in their little incubators at the end of the movie; near the end, when someone asks Spock if he killed Khan's crew (assuming he'd sent the missiles to Khan's ship with the crew in them) he tells them something like "I would never do something so heartless" and this is folowed by a shot of the crewmates, still in cryo-sleep.

I just thought it was interesting that the idea of keeping someone in an eternal sleep/coma is more humane, more comforting than the idea of putting someone to death. Spock didn't blow up the crew but would it really have made a difference if he had? I suppose the difference is that there is a potential for the crew to wake up; they are not dead, they are still alive. 

But the end of the movie makes it pretty clear that noone has any intention of ever waking any of Khan's crew up. They are stored away.

So why is there such a big moral difference between killing them and keeping them in an eternal coma? There is some degree of debate about what being in a catatonic state means in terms of whether the person is still 'awake' in their own heads, able to think and imagine and remember but not perceive the outside world. If this is the assumption, then I suppose there is a kindness in keeping the crew members in their 'eternal sleep'. 

But being in a coma isn't the same as being asleep, and therefore isn't there the implication that they are not 'awake' at all but fully dead to the world/vegetative? And if this is the case, would keeping someone eternally in this state still be morally superior to killing them?

This is obviously a big argument re; euthanasia and comas and switching off machines. But there is a difference in that the crew in Star Trek are intentionally kept in this 'state' but can be awoken from it at any time. Despite the fact that this will probably never happen, does that mean it's still better than being dead? It's all about contingency; if you could choose to be kept eternally 'asleep' or just killed, you might choose to sleep as there would still be the slim possibility that you might one day be awoken and organisms cling to life with every chance they get.
(there's a parallel with cryonics here, in which people are frozen at vast costs in the hope that future technology will be able to 'resurrect' them, although the variables in that situation are different, relying on future advancements rather than random chance/human curiosity or need)

Is eternal sleep morally superior to death? If you could choose between them, between eternal sleep and death, which would you choose? It shouldn't really matter as they are essentially the same thing, although there is a certain promise to the idea of 'sleep', the lack of finality and the potential(however improbable) for a return to awakeness that might lead people to choose eternal sleep. 

Still though, if beings are kept in comas rather than being killed, this isn't morally superior in any real way unless there is the intention of waking them up in the future. Otherwise they are dead to the world either way.