While both methods can work, I fall firmly into the "writing with a plan & purpose" category. Inspiration is a fickle thing, and I can't afford to wait around for it to strike.

In my career I've approached songwriting the same way I would if I were writing a daily column for a newspaper. You aren't always inspired, you don't always have a great idea to write, but you write something anyway. Interestingly enough, and this is my experience only, I've found that the more often I make myself write, the more often that fickle inspiration thing does actually happen. I get more, and better, ideas from working through and around ideas that aren't as strong. Waiting for that great monumental idea has always been a recipe for writer's block in my experience.

The great thing about songwriting is, there isn't a one-size-fits-all model for it. Everyone has to follow their own path.

http://www.rogerbrownsongs.com
http://www.nashvillesongwriters.com/roger-brown