Is it preferable to take airport bus or metro to the city center from the airport to avoid expensive taxi cost?
Sasha's right...the airport express bus is the way to go. I used the Aerocity door-to-door shuttle in late 2009 before the express bus was in service...that used to be the best way to go. It's still a good option with four or more people...very cheap that way, and you can preschedule a pick up right at your hotel door for your departure day...can all be booked online before leaving the U.S. Still might work for some travelers. But all other things being equal, I will use the bus next time and walk the 25 minutes up to PdS from Atocha.
We took the airport bus and it was easy, comfortable and inexpensive. If your hotel is not near one of the stops you can take a short cab ride from there. I think if you take the metro you have to change trains, depending on where your hotel is it may be one or more changes. Also I saw warnings on Tripadvisor from locals who warned about pickpockets on the metro. Not something you want to worry about with luggage, etc. coming from the airport. That is why we took the airport bus and are glad we did. I think it was 2 euros each.
Thank you for the advice!
Ruta, it is also worth checking as to whether your hotel has an airport shuttle. We were in Madrid in April, and found out late in the game that Hotel Atlantico does have a free airport shuttle which runs at specific times of day. So our plans changed from Express Bus to the hotel's shuttle.