When I made my first couple of quilts, I used poly batting. I have since discovered 100% cotton batting, and cannot believe the difference! Maybe it's just me, or maybe other things have changed as I have gotten more experience, BUT the cotton just seems to work better for me.
Also, I have learned the hard way that sewaholic is right -- pins, pins and more pins! And a walking foot is an absolute necessity when stitching in the ditch.
Marcia said it really well:
the best quilts have thinner batts and then "puff up" some when we wash them
Hope it turns out well in the end!