Keep the jetlag in mind if that is an issue for you personally. I believe four and four is just fine. Obviously, you could always find arguments for staying longer in a particular place. You said you liked "local attractions, walking tours, shopping, and taking in the cities" and this is exactly what you can do with ease in those cities. Getting a taste of them, seeing stuff you feel suits you best. And in case either city runs out of attractions for whatever reason, there's still some really nice day trips and tours to do.

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BigAl, you seem to get the gist though and thx big time for the correction. Love it!

MaloBalo, thank you...Very logical advice.

I think we'll do Barcelona 4nts, Madrid 4 nts.

Thanks to everyone!

Malo

It was the only humane thing to do.

I agree, don't do any more than two bases in 8 days.

However, a potential option for you if you want to get a flavour of Andalucia would be to day trip from Madrid to Cordoba or Seville (I would recommend Cordoba as Seville needs more time to fully appreciate).

It's less than two hours each way so if you were willing to get up early you could have 10-12 hours in Cordoba and see alot of what it has to offer. Of course Cordoba is worth a few days but you may like this an option as a taster.

Thinking 4-5 in Barcelona and 3-4 in Madrid. You should pick one of Toledo/Segovia and Toledo is both larger, more important historically and more interesting. Segovia is a small town with an ancient aqueduct, sizeable house of worship and decent castle. Neither Toledo nor Segovia is worth dropping a day from Barcelona.

Also: compare the Mercado San Miguel in Madrid to La Bocqueria and report back - Big Al and I have a Big disagreement on this ;-)

Barcelona and Seville, definitely!
I have visited Madrid and Barcelona and I found that Madrid has the same stuff as Barcelona, but it is just bigger and much more crowded (plus it does not have beach). If you only have 4 nights/city I would rather go to Barcelona and Seville. Which are totally different in architecture, culture, food, etc. so you will get more different kinds of experiences than visiting only Madrid and Barcelona.