My suggestion would be Hobbs wool. It has enough loft that the quilting stitches will show, but is light in weight. I would choose Hobbs over Quilter's Dream mainly because someone on the QB had problems with bearding using the QD wool. If I can find that thread, I will post it here.
Cotton is relatively heavy. If you go with a really thin cotton it will be light in weight, but you will get very little loft (and a relatively flat result after quilting). Cotton batting does become very soft with repeated washings, and that softness is wonderful. If the flat look is okay for you (and it is consistent with the modern look of your quilt top), the QD cotton. Request is their thinnest, lightest weight. Here is a link:
http://www.quiltersdreambatting.com/dream-cotton.htm
You can get loft and lightness of weight in a polyester batting also. However, polyester doesn't "breathe" which means it can trap heat and moisture. Wool and cotton both breathe, making them suitable for both warm and cold climates.