Which era DA? 7400 or higher, and I am keeping the Univega. Older DA, I am probably cleaning it up, selling it, and using the proceeds to fund the Trek. This assumes size and fit are comparable. AS to rust, Trek had the most DURABLE paint I have seen on vintage steel bikes. I have found Treks that had been laying in the weeds for a few years, vines growing through them, all components rusted up, and the frame pristine.
Even if it is DA 7400, I would be tempted to swap components in that case. Really depends on condition of frames, components, rust, whatever.