You also have to factor the fact that when word gets out in the prison that you snitched to save 5 years, the other prisoners are going make your life hell, if not end it. Especially if your crime was gang related and a gang member is in the same prison.
Depends on what I snitched on. If, for example, it's the case that I'm a terrorist and gave the government valuable information on the terrorist organization in exchange for some immunity or protected custody, then it could play very much in my favor to turn myself in supposing I was highly likely to get caught and my ties to the organization were strained for some reason.
Also when you cut a gang-related deal you make sure to get put in a prison without any gang members in it, for your own protection.
An example of when it'd be a bad idea to turn yourself in is if you're involved in some sort of underground human six trafficking organization that has been involved with kidnapping underaged persons. Even if you were in it solely for the money, you'd still get labeled a six offender, you'd be constantly picked on and possibly killed in prison (even criminals hate these guys), once you've gotten out you'd be completely ostracized from society, job options would be limited, and you'd be the constant recipient of hate mail and acts of violence. If you were to follow the line of thinking in the OP as doing things from the "practical "whats-in-it-for-me" perspective, then it'd be wise not to turn yourself in.
The same situation can be spun though so it is favorable to turn yourself in: you're higher up in a crime syndicate that has a branch involved in human trafficking like above. You have no ties to it yourself, but have the dirt on it, and for unrelated reasons you are being charged with something (say, murder) and the above has never sat right with you (but you stood by because you didn't want to lose power). Now you can trade that information on the trafficking for a more lenient punishment/protection.
So like I originally said, this cannot be given a blanket statement, if doing as the OP originally proposed of doing whatever is in your best interest, as there's too many crime and too many considerations to factor in for a simple answer to be given.
BTW: I enjoy a good game of prisoner's dilemma if you haven't figured that out
