If you plan on powering something that draws more than 1-2A of power, then, yeah, go with switching supplies. But for 150mA... A good ol' 7806 will work great. Or you could get an LDO. A bit more expensive, but it's more efficient.
Since the servo won't always draw 150mA, when it draws less, the voltage drop on the resistor will be less (Ohm's law again) and it will supply your servo with more than it can take, thus frying it.
Bottom line: go with a 7806.