If i have to choose one of these; which is best to visit for a first timer? 2 days in one of this city sufficient? from there i would continue to Andalusia, Seville,etc
thanks.

It depends on what you want to see and experience, which only you yourself can answer.
Barcelona www.barcelonaturisme.com
Train Madrid to Cordoba and Seville www.renfe.com/EN/viajeros/index.html
Flights Barcelona to Seville www.vueling.com
On the Respective destination forums Top Questions, you will find further information for your visit.
Accommodation www.venere.com
tancp:
For a first visit, I would choose Madrid, because it is Spain's capital and has the most sights. While in Madrid you can do a day trip to Toledo and another to Segovia. Read about these day trips in the top questions of the Madrid forum.
TQ very much for your input. Appreciate it.
I would also choose madrid, it is a shorter train ride to seville and a more interesting place imho than barcelona. nevertheless 2 days is not enough for either one.