Some people claim (now Jimmy Buffet's in my head) that aspen is "safer" than pine. No idea why, but maybe because there's less aromatics? I use large pine flakes because they're affordable, available, and the chicks can't eat them unless the bag's full of fines.
Hemp is vastly superior in odor control, but both of the broods I tested it with had blood in their stool shortly after switching. This could have been a coincidence, but I'm a bit reluctant to push my luck. No one else seems to have had this issue.