Ford Motor Company's current incentives include $3\mathrm{.}9$ percent APR financing for $84$ months or $$1,500$ cash back on a Mustang. Let's assume Suzie Student wants to buy the premium Mustang convertible, which costs $$21,000,$ and she has no down payment other than the cash back from Ford. If she chooses the $$1,500$ cash back, Suzie can borrow from the VTech Credit Union at $5\mathrm{.}9$ percent APR for $84$ months $($Suzie$\text{'}$s credit isn't as good as Prof. Finance$).$ What will Suzie Student's monthly payment be under each option? Which option should she choose?