Well, assuming that a) I lived in somewhere with the death penalty - America's system, I'm assuming this poll is based on - and b) I'd actually committed said heinous crime for which I've been convicted, then I'd choose prison. Given that I've done something bad enough to warrant life imprisonment, I obviously need some serious help.
What a lot of people who complain about nice prisons miss is that it's not about punishment - all that does is create resentment when criminals are released. It's about (or should be about) rehabilitation, to make sure that when the sentence is over they are fit to rejoin society as normal people free of whatever issues drove them to crime in the first place, whether that be mental issues, drugs, desperation or whetever.
Of course, life sentences (real life sentences,not 25 years, cut to 15 for good behaviour, with a chance of parole) tend to miss this entirely, as with the death penalty. That seems to me that it isn't trying to help the guy who obviously has some major issues with his head; it's just the state retaliating - a 'you took their life, so we'll take yours' situation.
Some people, those who are just genuinely nasty pieces of work with no realistic hope for redemption, would be better kept away from society. These are the people who need life sentences - but if they're going to be isolated from the public anyway, why get up in arms if they get tolerable-to-nice living conditions? Mental institutions would be a better place then prison for true lifers.
But then I'm getting a bit off topic now, so I'll stop.
