Are you planning to work on these yourself? Getting the Miyata cleaned and tuned at a bike shop could easily cost $100+. Riding older bikes is only cost effective if you have at least a small bit of skill on maintaining them yourself and a few tools. It doesn't take much but it is something to consider.
It sounds like the Peugeot doesn't need any work but it will be easier to find replacement parts for the Miyata. The Peugeot used parts that are now obsolete while parts are still readily available for the Miyata. I'd keep the Miyata, sell the Peugeot and clean and tune the Miyata yourself. You'll save a bunch of money and learn enough to be ready to maintain the bike in the future.