Hi. In the market for a new system or two and hoping that you can help settle a discussion between my wife and I.

Our current system consists of two 10 SEER gibson units about 10 years old. Downstairs unit is 2 tons and upstairs is 2.5 tons. The downstairs unit has worked fine for the year that we have lived in the house. The upstairs unit recently developed it's second siginificant freon leak in the condensing unit. We are going to replace with a new R-410a unit rated at SEER 15 or better. Additionally, we have had two load calcs done with the same conclusion - our current units are not even close to the correct size. The load calcs suggest a 3.5 ton unit downstairs and only a 1.5 ton unit upstairs
Would you recommend replacing both units at the same time? What are the key factors we should consider in making the decision to replace the downstairs unit?