No. The first states that you would like to reciprocate. This is a normal thing to do. The second is stronger and less frequently appropriate. Don't say it this way unless you believe the person merits a reward. A "reward" is like a prize, sometimes even money. You might embarrass someone or even insult them by saying you plan to reward them.

While both sentences express a desire to reciprocate or acknowledge a favor, they have slightly different tones and implications. "I'd like to return the favor" is a straightforward and commonly used expression indicating a willingness to do something in return for a favor someone has done for you. It is a positive and appreciative statement. On the other hand, "I don't want to leave your favor unrewarded" suggests a sense of gratitude and a desire to acknowledge the favor by providing some form of reward or repayment. This statement implies a more intentional effort to show appreciation. In general, you can use them interchangeably in many contexts, but be mindful of the subtle differences in tone. If you want to convey a straightforward intention to reciprocate, the first option might be more appropriate. If you want to emphasize a sense of gratitude and the idea of actively rewarding the favor, the second option might be more fitting.