It's a good question and it depends on the story. For What Goes on Tour it was character. I had the idea for a rock star whose rock star persona wasn't who he was at all. I liked him so much that I then needed to figure out what kind of story would work for him and who would be his ideal heroine.
For All that Sparkles, the plot came first in the form of a dream. I had a rich girl and a poor boy and they weren't allowed to see each other. Then I had to figure out who these people were and what they wanted.
For the other two books in The Texan Quartet, the characters definitely came first. Piper and George had appeared in the previous two books and so I knew who they were. And in each case, their perfect match walked on to the pages of the book before. With my characters chosen, I needed to decide what their goals were and why, so I could come up with their story.
I guess this question really goes hand in hand with where do you get your ideas from? I've had a number of story ideas from dreams, or an idea will spark while I'm watching a documentary or reading a book. Those ideas are generally plot driven and then the characters come later.
With my latest series, The Flanagan Sisters, the first book came from a plot idea I had, but because I knew that I wanted to make it part of a series, I needed to create the three sisters before I started. So I sat down and ran through my character profile checklist and developed three characters. I wanted to have an idea where the series was going so I also developed three heroes for my sisters and the plots for books two and three grew from there.
So really it doesn't matter which comes first, what matters is when you have finished, you have an interesting story and characters the reader can connect with.