The man pages of gcc on Mac OS X say that they’re synonymous:
-mmacosx-version-min=version The earliest version of MacOS X that this executable will run on is version. Typical values of version include 10.1, 10.2, and 10.3.9. This value can also be set with the MACOSX_DEPLOYMENT_TARGET environment variable. If both the command-line option is specified and the environment variable is set, the command-line option will take precedence.