Same machine, two ways to pay. I have the cash, but it is also our only buffer. Which one?

Option B is the more prudent choice because the cash is explicitly the only buffer. Paying the full $4,000 now would still leave about two months of expenses, which is below the commonly recommended 3–6 month emergency reserve and would irreversibly weaken the sole safety net. The installment plan costs about 15% more—roughly $600 over two years, or about $25 per month, implying an effective annual cost of around 13–14%. That is a real and relatively expensive financing cost, but it functions as affordable liquidity insurance: it keeps the full emergency fund intact, preserves flexibility, and avoids exposing the household to a severe downside such as a major repair, medical bill, or income interruption. In a stress scenario, spreading the payments can also preserve a longer survival runway than draining most of the cash at once. Option A has genuine advantages—saving the $600 premium, obtaining outright ownership, and avoiding a fixed monthly obligation—and it would be reasonable if income were highly stable and cheap emergency credit were reliably available. Neither is stated, and the framing emphasizes scarce liquidity. Because avoiding catastrophic liquidity loss is more important than optimizing a modest interest cost, Option B is the sounder decision.