If I went to a long-armer, I'd use what they recommend. If it is from what you have I'd use whatever is thinnest. If you use a bonded cotton I'd do what jacqueck recommends. I did my first fleece backed project about a year ago and did not use any batting, would have liked at least a thin poly batt. Squish factor is nice and although it was not an issue in my project, the fleece was very dark and could have shown through a different top.
I don't get as many quilts finished as I do pieced tops because I do my own quilting and it is not my favorite part of the process. I typically use a rather puffy poly batt and I always fluff it in the dryer before laying it out.
Warm and Natural is sort of the default right now for modern quilting. It's very heavy to me, and partly from design and partly from ability I don't do a lot of dense quilting which is perfect for it.