Just list out the pros and cons.
I think it's really important to factor in your psychology and feelings toward the subject as well.
If you hated it or even just disliked it and conversely did bad (aka me in math), repeating it, despite what kind of motivation you might of mustered up, will probably be a bad idea. I'm not saying its a sure thing but if you hated it once, doing it again wont be much better.
Likewise, some people are more or less able at certain subjects than others for various reasons (motivation, likes, background, sheer raw ability, etc).
Sometimes, you just gotta know when to fold your cards.
As Pi said, unless you got lower than you were expecting and it wasn't because you hated it or just weren't good at it, probably shouldn't redo it. If you did get lower than you expected and already know all the stuff, even then, it might be a bit boring to do it all again. At the same time though, it certainly would be easier than picking up a totally new one.
All about your personal pro's and cons.