Here are some websites with batting information:
http://www.bellaonline.com/articles/art10201.asp
http://www.planetpatchwork.com/battjunk.htm
http://www.dreamweavers-quilts.com/q...atting-review/
and an old Hobbs batting chart:
http://www.how-to-quilt.com/articles/quilt-batting.pdf
My personal preference is Mountain Mist Blue Ribbon cotton batting, which is a very traditional batting that has been around for decades; I like the crinkled look and this batting gets softer with every washing so that years later it's even softer than when originally made.
Poly and wool have their uses (wool, for example, is exceptionally warm and light), but I tend to avoid them because of the potential of bearding problems on dark fabric.
For a higher loft batting that remains soft and light, I like Hobbs 80/20.
Many people like Warm n Natural, but years ago when I used it I found it has a stiffer drape than other cotton battings (probably because it is needle-punched through scrim), so I use it only for wallhangings now (exceptional stability).
All of the above are my choices for machine quilting; for hand quilting it's important to choose a batting that is easy to hand needle. Quilter's Dream (both cotton and poly) and Hobbs 80/20 are better for hand quilting; wool can be excellent for hand quilting also.
As others have said, it becomes largely a matter of personal preference.