Hi,
I prefer metro because of possible traffic jams. You use to wait less for trains than for buses. But, of course, it depends on where and from are you going, it could be a better combination on metro or bus.
The price is the same: 135 pesetas ($0.70) for one ride. But in metro you can do all combinations that you need with one ticket, while in bus you need one ticket for each bus you take.
There is a 10-ride ticket ("metrobus") for 705 pesetas ($3.70) that is valid both for bus and metro.
And then, there is a month "bono" for about 5,000 pesetas ($26.30). It´s cheaper if you are below 21.
Saludos.