They mean almost the same thing, but there is a small variation in meaning, often depending on context. "Spending time" often means you are engaged in a general activity: "On weekends I spend time playing sports". "Investing time" often means you are focused on a specific goal or outcome: "In order to pass the exam I invest time every night studying". Now, you could correctly say, "I spend time studying", but you would typically NOT say, "I invest time playing sports". I hope that helps.