Hey ilanit!

When I lived in Madrid it depended on many factors when to take the bus and when the metro. If you´re new in town, you´ll probably find it fascinating going by bus an seeing how "civilized" spaniards drive;-)), all the nice buildings, fountains (La Cibeles-Hala Madrid!), etc.etc. On the other hand, the metro is pretty fast. You also should consider your gender. I don´t know if a lady would like to walk through these long underground-tunnels by herself at 12.30 a.m. And believe me, some are veeeeeery long and veeeeeeeery lonely at night.
Hope I could help.
Chao
Werner