Watch the Sing The Melody First online guitar lesson by Paul Brown from Smooth Jazz Handbook
I like to sing the melody I'm going to play first. The reason for that is again, the restraints of the guitar. You get into these patterns and you get into finger positioning - where on the neck sounds good? When you're just listening to music and singing, those issues don't become a problem. Usually the restraint and the melodic structure is brought out a lot easier when you try to sing a melody as opposed to try and figure it out on the guitar.; especially if there is a lot of chord changes and different keys. Sometime if you try to react to a key change, it's easier to plan what you are going to do. It really does open up your mind. Anything is possible in your brain, when you get to that point where you think of something and then bring it to the guitar that is really an advanced place to be. Even the greatest musicians on earth have difficulty with that.