Which Is Better?

I have been trying to answer a question that only leads me to more so maybe you can help.  In chess it is beneficial to think several moves ahead, however if the correct move is not seen or you cannot see several moves ahead, should you play defensive or should you eye an unprotected piece and take it down?  Which is better?  Does it depend on the board?  Your opponent?  If you can't see several moves ahead and you're lost during the middle game which is the best guideline to follow?  Thanks.