I’d endorse AlanC’s comments.
If you’re on limited time I’d stick to travelling between places on motorways.
Spanish motorways are usually cheaper than Portuguese, or free, but both are fine networks. On that basis if you’re going to the south or middle go for Salamanca and then head towards Cuidad Rodrigo on the A62 or Merida and Badajoz on the A66/A5. I don’t have any experience of going further south. Salamanca is fantastic but you need to stop two nights and take a full day.
The northern route is worthwhile if you want to sample the North Coast scenery and to take in Santiago de Compostela. We enjoyed Santiago, especially chatting to some of the pilgrims and finding out their motivation. On southwards from here into Portugal the motorways are fine. If the weather is good, though, you might be tempted to saunter through Northern Spain, which could blow your plans apart!
You could always go one way and then come back the other depending on your schedule.
Make sure you go into Portugal somewhere you can pick up a motorway pass. I did a post a while back elsewhere in the forum on this