We chose batts.
For this garage project, Johns Manville formaldehyde-free encapsulated batts were selected. Because the insulation is wrapped in plastic, the homeowners and friends who helped install the insulation experienced minimal itch and dust associated with traditional kraft-faced (paper-covered) products. (Safety glasses, nuisance dust masks, and long sleeves are still recommended.) According to the company Web site (jm.com), the poly-faced vapor retarder is twice as resistant to moisture as kraft-faced products. Thus, no vapor barrier was required.
This project qualified for federal tax credits for energy-saving projects. Before you begin your insulation project, check with your utility company to see if tax credits or rebates are available.