Option 1:
I suggest you to pay off the car loan that is with 6% interest rate:
Unless you can use the money to invest something else like, stock that can generate more than 6% return. If your return is 10%, you make 4% profit. The 10% is taxable, but you still make profit. However, the future stock market will be very volatile. If you are not sophisticated investor, you better don’t touch it. If you put money to CD, the max is about 5.5%. So, you lose 0.5% to carry the loan, plus the 5.5% CD profit is taxable. So, you actually losing 0.5% + tax.

Option 2:
I don’t suggest you to use the reward credit card to pay off your car loan:
You did not mention the interest rate and reward % of your rewards credit card. So, it’s hard to compare. But I know the max reward % is about 2%. Sometimes, credit card offers very low introductory rate, but it will go up tremendously, like over 20%!