I really feel that the option both should be up there, but in its absence, Ill definitively vote rehabilitation.
You see, the idea of relentless punishment is nothing new. In fact, in ancient eras, people had no sense of proportion. It was like: Fool? So you stole my apple? Well, now I have ripped off your arms, raped your wife, killed your children and set your house on fire.
Of course Im just illustrating, but you can see that line of thinking in countries that to this day applies severe punishments like amputations and whipping.
Anyway, criminality still exists in such countries. Because criminals only commit crimes because they believe that they wont get catch. If they believed that they would have to pay for it, they wouldnt do it. So, making them punishing places wont diminish criminality, the same way that making them rehabilitation places wont boost it.
What brings me to my next point. See, criminals are citizens too. And, like other citizens, they deserve society care. The reason why society is entitled to apply penalties is because its supposed to be better than the criminals. If its just angry and vengeful, it wont be better. And will be failing its duties.
And this goes to all criminals, even the most dangerous ones. If they cant be cured, they should be kept isolated for as long as necessary forever, if it gets to that but with other finalities than punishing, pure and simple.
I understand the anger society feels with violence. I was victim of violence too, once. Nonetheless, I still think that rehabilitations the way to go. Because, seriously, what good does plain punishment brings? Nothing but the satisfaction of a primal sense of revenge.
Rehabilitation, on the other hand, is to give someone the chance to be a productive citizen again, and, even, try to repair the damage that he/she has made.
Anyway, just my $ 0,02.