Not specific for converters, but if you want to power it with 5V, use the 5V. One catch, DON'T connect the usb to the computer when you feed it with 5V. Or if you do what that (and I don't blame you), take a USB cable, cut away the sleeve and cut the (probably) red power wire only.
But again, servo's are power hungry, think about 1A+. And even then you might want/need to use a decoupling diode for the Arduino power.