I would recommend retaking the GRE to improve your quantitative score -- 39th percentile could cost you admission. Assuming you have the time to study and retake before your app deadlines, you should retake. If it's too late, I would also consider putting off applications until the next cycle so you can get all your marks up to scratch. If you're gonna go with it, I wish you the best of luck.
In regards to your statement, you can include things already in your CV. The difference is that in the essay, you can tell the committee what the experience meant to you and what you learned from it -- things they can't deduce from just a resume. An important piece of advice regarding essays -- "Show, don't tell". If you want to demonstrate your leadership abilities, don't say "I am an effective leader". Instead, share an experience that proved your leadership abilities. And you should definitely try to tie in your letter with the school you're applying to, even down to specific faculty interests and papers. It doesn't hurt to show them that you are aware of the work they're doing, and how it plays in to your own ambitions.