A lot depends on what time of year you intend visiting.
Ia gree with Cathy, go to Andalucia. Northern Spain is wonderful too but better a bit later in the year.
In March it has to be Andalucia - much better weather than in the North - although 6 nights will only give you a tiny taste of this fascinating region.
Save the North for a future Summer visit.
Great suggestions!!!! Then Andalucia will be!
Granada and Sevilla as main spots ... any other tips about these places? I will be driving.
Buy your Alhambra tickets week in advance - can be done up to 3 months ahead - at www.alhambra-tickets.es
Don´t just stick to cities - have at least one night in the countryside to get a different perspective.
I honestly wouldn't bother with driving. With only 6 nights this will barely give you enough time to see Sevilla, Cordoba and Granada. A car would be great for exploring but you just don't have time and it will end up being a hassle driving and parking in the cities.
Route planner www.viamichelin.com
Car rental www.auto-euope.co.uk
In Ronda check accommodation in the old town, Espirito Santo
and Granada I can confidently recommend the Hotel Abadia, with parking