For me, I sometimes find a tune whirring round in my head that I then put words to, sometimes I find words buzzing in my head and put a tune to them sometimes the two come simultaneously. No hard and fast rule. Maybe some are better than others, but I dont think that any one way is better than another, its just how the mood takes me. The best songs I have written have been about those events in life that stirred me emotionally enough to want to write something as a statement about them.
Cheers, Lucy.