I think that prison should be a punishment first, with rehabilitation being offered to those individuals who it is likely to do good. I don't agree with the Senator, because this isn't capital punishment, it is letting a criminal get away with his crimes without paying his debt to society. We don't give criminals the option of where and when they go to jail (normally), nor should we give them control on when or how or if they die. My personal opinion is that prison, for the most heinous of criminals, should be like Dante's inferno.
As for the DP, I'm on the fence. I don't believe it is much of a deterant for the worst of offenders: the serial killers, the psychopaths, the drug induced crazies. Therefore, its real purpose is either a means of punishment, or to stop recidivism. For the first, as stated above, I think death is too good for them. For the second, I'd have to be convinced that there have been significantly more victims killed by repeat offenders than there have been innocent people wrongly accused.