Happy to be of help!
The route you take really depends on the way you intend to travel. We hired a car for the week and based ourselves in Carcassonne, as the major focus of our trip was to explore the fabulous Cathar castles in the surrounding region (a Christian sect who were persecuted by the Catholic Church in the 13th centuries). I've seen a lot of castles in my time, but Peyrepertuse is the most impressive fortification I have ever experienced, and is perched on a dragon's back ridge high above the surrounding region.
We also visited the enchantingly pretty coastal town of Collioure - home to many Impressionist artists around the turn of the 20th century and famous for its delicious local anchovies (thanks for that tip, beausoleil). There are also very pleasant 'second division' cities towns such as Narbonne with interesting history and good bird spotting in the lagoon, and Cite de l'Espace in Toulouse will allow you to rediscover your inner child (you get to experience what a moonwalk would feel like, clamber through a relica of the Mir space station, and after 7 hours, we still hadn't experienced it all).
If you don't hire a car, then both France and Spain are both well served by buses and trains. On a previous trip, I based myself in gorgeous Avignon and explored the surrounding region by public transport. That entire area is steeped in Roman history - Orange, Nimes and Pont du Gard are just three stunning examples - as well as fascinating Papal history (Avignon was the seat of the Antipope during the Papal Schism that bizarrely gave rise to two popes). I even went swimming (if not snorkelling) under at iconic aqueduct at Pont du Gard ... but then that was early October
Barcelona is a vibey, feel good city that's proudly Catalan, and feels very different to most other Spanish cities. I think you could spend months there without running out of interesting things to do, and visiting out of season, you wouldn't have to contend with the crowds that besiege it in warmer months - I visited in early November, and it was lovely. It's worth visiting for Gaudi's architecture alone, and the Sagrada Famiglia cathedral is truly one of the wonders of the world.
At the time of year you are planning to visit, I would stay south for the majority of the time, because the weather will be better, but in my opinion, you really can't visit France and miss out on visiting Paris. Keep an eye out on the SNCF rail site, which offers regular special fares that are terrific value.
Hope that's enough to get you started, and feel free to revert with further queries. Researching and planning is part of the joy of the travel experience
Thank you so much for taking the time to write such a helpful experience!I appreciate all your advices and will take what you've said and plan ahead with lots of fantastic information!