is it better to pay the optional final payment, part exchange the car or return the vehicle and get a new one?

It's not clear what you're asking. "Optional final payment"? Are you asking about paying off the residual value on a lease at the end of its term? "Part exchange the car"? I don't even have a guess what that's supposed to mean, because that's not a thing, neither in car lease expirations or trading in a car you've bought. 

All I can say is that if it is a lease you're asking about, it's financially advantageous to opt to pay off the stated residual value of the car as stated on your lease contract, the residual value being what the lessor predicts the car will be valued at at the end of the lease, if the actual value of the car at the actual end of the lease turns out to be higher than the residual value, which can be because you put far fewer miles on it than allowed for by the lease contract or because its value depreciated less than was expected. So you'd look up what the car is worth in NADA or in Kelly Blue Book to see what it's actual value is, and if you can sell it for more than the residual value on the lease contract or if it would cost you more to buy that exact same car to replace it than what it'll cost you to just buy it by opting to pay off the residual value on the lease contract, than you should opt to pay off the residual value on the lease contract. If not, don't.