Both perform better than batts and are less expensive than spray polyurethane. Neither is perfect.
Fiberglass-batt insulation is inexpensive but difficult to install well. Framing members aren’t always spaced perfectly, and it’s awkward to fit batts into irregularly shaped cavities and around electrical wires and boxes. Because few insulation installers have the patience required to install batts carefully, insulation performance suffers.
[To read more of Martin Holladay’s article from the December 2010/January 2011 issue of Fine Homebuilding, click the link below.]