What arguments would support the claim by Martin and Duke for the return of the deposit?

Daniel Martin and John Duke contracted with J & S Distri-
butors, Inc., to purchase a KIS Magnum Speed printer for
$17,000. The parties agreed that Martin and Duke would send
one-half of the money as a deposit and would pay the balance
upon delivery. They also agreed to the following provision:
In the event of non-payment of the balance of the
purchase price reflected herein on due date and in the
manner recorded or on such extended date which may
be caused by late delivery on the part of [the seller],
the Customer shall be liable for: (1) immediate
payment of the full balance recorded herein; and
(2) payment of interest at the rate of 12 percent per
annum calculated on the balance due, when due,
together with any attorney’s fees, collection charges
and other necessary expenses incurred by [the seller].
When the machine arrived five days late, Martin
and Duke refused to accept it, stating that the company
had purchased a substitute machine elsewhere. Martin and
Duke requested the return of its deposit, but J & S refused.
Martin and Duke sued J & S for the return of its deposit.
J & S counter claimed for full performance of the con-
tract, seeking an order that Martin and Duke accept deliv-
ery of the KIS machine and pay the entire balance of the
contract.
a. What arguments would support the claim by Martin and
Duke for the return of the deposit?
b. What arguments would support the claim by J & S for full
performance of the contract?
c. Who should prevail? Explain.