I take it you are looking at a PCP? What is the difference in the final payment, and what is the difference in the APR?
New cars typically have better % final values than used, in which case the extra payments (for the used one) may lead to more equity at the end to roll into the next car. Don't forget though that it will be around 12m older, and therefore the values are not directly comparable.
New cars typically also attract manufacturer discounted APR's, which is obviously good, and reduces the amount spent in interest.
What is the cash price difference between the two cars?