Your argument is like comparing the number one (or even the number 100 or 1000) with infinity.
A finite sentence, even to the end of their life, is just that-- a finite event.
Whereas death is infinite (well, may as well be-- yes I know the Earth will eventually be swallowed up by the sun one day, but...)
I simply do not see that the two are even close in comparison.
Re: Individuals choosing death? I think that would/should always be acceptable-- I do firmly believe in every individual's right to self-terminate, if they so choose.
I do not think this right should be removed, regardless-- the single exception if you can make a valid case that the individual in question is not of sound mind (and the definition of "sound mind" is left up as an exercise for the student, or perhaps for a different thread altogether).
Furthermore? I think it would be Humane, to always provide a safe, effective and entirely painless method of self-termination for anyone in prison for any reason, should someone wish to avail themselves of it.
To force someone to continue to exist, when they have determined they would rather not, is not only immoral, but is Evil with an E.
But that's just my opinion, of course...
And it's absolutely your opinion to make. I'm not taking that away but I have to note that that is an opinion.