I would use the bikes you have. I had an 89 520 and hated it, felt like a slug. I have an 85 720 and love it. Converted to 700c wheels and albatross bars. Whichever bike you like the most upgrade to what you want and keep or sell the other. I have many bikes set up for different purposes, but do have overlap. My 720 is set up as light as possible, just a saddlebag. The Schwinn passage is set up similarly with Soma Oxford bars but has racks front and rear and fenders. Both ride similar, if I need to haul more or rain in future, ride the Schwinn, if clear and do not need to carry anything, the Trek. I have a dyno-hub I can swap to either pretty quickly. The trick is to define exactly what you need and work to that goal. In the past it seems I ended up having all of my bikes outfitted the same and would end up selling one or more and buying others just to do the same thing. I am getting more disciplined as I age and am lucky I really enjoy all of the bikes I have, even if I do not ride some very much.
Enjoy the ride,
Tom Palmer
Twin Lake, MI