Re-imaging is the process of saving the user state on a computer, installing a new image on it, and then restoring the user state.
Bare Metal deployments definition:
Bare Metal deployments are installations of operating systems to targets that either have no operating system installed, or must be re-installed without preserving any existing data or settings.
Based on the definitions above, if you don't plan to preserve any data or user state of the current existing installed OS, you should opt to use Bare Metal deployments. Another thing to note is that Re-imaging only supports the same architecture. You can only upgrade the OS or install a later service pack, but you cannot downgrade architectures (64bit to 32bit) or OS (Wind7 to WinXP). You cannot also change from a server (Win2000) to a desktop (Win7).