10 Days: Spain, Portugal, or both?

With 10 days I would pick one country. Connections between the two are tougher than you'd think, given the geography. Seville and Granada are magical, I would definitely say you need to see them to have a good picture of Spain.
Or you could spend your 10 days in Portugal if you prefer it.

Just got back from Lisbon and Spain. Lisbon was forgettable except for the aquarium and Sintra. You do not want to miss Granada-the Alhambra and Royal Chapel. Toledo and Segovia are very interesting. If you don't go to Lisbon, you could spend a night in Toledo.

I agree with Mira and Woinparis, pick one country and save the other for another trip.

If you decide to go to Spain, look at a map and figure out the best route to take.
Since you only have 10 nights to work with, I'd choose Madrid or Barcelona, but not both.

I'd consider something like this:

-Fly to Madrid 3N- possible day trip to Toledo or Segovia
- Córdoba 1N
- Granada 2-3N - book Alhambra tickets 3 months in advance.
- Sevilla - 3-4N
- Fly home from Sevilla.

Another option would be:

• Fly to Barcelona 3-4N
• Fly to Granada 2-3N
• Sevilla 4N with day trip to Córdoba
• Fly home from Sevilla

Late September/ early October is a nice time to visit Spain, but I wouldn't say that it's " less touristy" especially since Spain in a popular destination. October is still considered high season.

Happy planning!

We just got home from our trip and we did this route above to a degree (no Cordoba and we flew home from Madrid bc we were on miles):

Fly to Barcelona 3-4N
Fly to Granada 2-3N
Sevilla 4N with day trip to Córdoba
Fly home from Sevilla

You could consider dropping one of the nights from Granada (2 nights was enough for me) and spend one of the white hill towns on your way from Granada to Seville (you'd have to drive).

Given the places you mentioned, that's a lot of ground to cover in 10 days, so I'd pick one country. Had all the cities been in southern Spain (Sevilla and west through Extremadura) and southern Portugal I would have said possible. You don't want to spend so much time getting to places that you don't have time to enjoy them when you get there. In general, there are fewer tourists in October and hotels are cheaper than June-August. However, the major tourist sights are still going to be busy. Some of the lesser and off the beaten path places won't be as busy.

Going for 2 countries means you spread yourself quite thinly. With only 10 days I think you need to choose either Barcelona or Madrid as your entry point. I'd be tempted to go with Madrid and leave Barcelona for a future trip. So yes I am totally echoing my friend Priscilla's suggestions above.

Arrive in Barcelona or Madrid (4N)
Fly to Granada (2N)
Train to Sevilla (4N)
(with day trip to Córdoba)
Depart from Sevilla

I was planning to do Spain & Portugal in one trip & my wife wouldn't let me go over 2 weeks. I quickly realized Portugal was definitely a full trip on its own. We did 14 days in Portugal by flying into Madrid, renting a car, driving a loop around Portugal and back to Madrid to fly home. (We enjoy driving so we can stop along the way.) We are booked for Spain in October (Toledo-2, Cordoba-2, Granada-3, Ronda-2, Seville-3, Guadalupe-1, Salamanca-3) so you can see there is no way to do both in one trip. I say all of this to say do at least two trips. Portugal has so much to offer. Our Portugal itinerary is below. We also try to check out places between night stops so some are included.

Évora (2), Lisbon (4), (Day to Sintra), Tomar (4), Madrid (2)

When we were driving to, and while in, the Tomar area we visited Obidos, Alcobaca, Fatima, Batalha & Malvão. Tomar is a great little city with a lot to see and do. We also stopped in Coimbra and toured the University & city on our way to Porto. The drive back to Madrid allowed us to experience the Douro Valley as well. Hope this helps.