I would undo the bottom radiator hose. The plastic drain plug never works because it is usually stuck and you end up breaking it off, but additionally you will get more fluid out using the bottom hose because it sits lower than the drain petcock. Don't forget to remove your upper radiator cap whener draining the radiator from either site. I know some oil change shops are now offering to siphon radiators for a fee, but remember gravity still holds all the particulate in your radiator. Gravity flushing is the only way to go and by removing the lower radiator hose any crap in the engine block will also flow out. End result you get a good flush.
