As with anything else, it depends on the circumstance.
If you are playing a deck and you notice that games are often won/lost by one life, take Mark of Mutiny. If you can kill the creature before it gets returned to your opponent (for example, if you have sacrifice a creature costs), Mark of Mutiny is obviously better. If you just want the creature to use an ability it has, or as a surprise attack, Act of Treason is better.
I don't think anyone can help you more than that unless you post your decklist.
