I agree with David in that I approach them differently. When I'm writing something for a film score I'm working on, it's obviously focused on the building of the arrangement.

When I'm writing something with lyrics, I'll usually start with some nugget of the chorus or a verse, and write it out from there, but usually always with a melody in mind.

However, there are times when I run across a chord progression and grabs me and I'll set that aside and write lyrics to an existing arrangement, but this is the exception and not the rule, at least for me.