should i return my mmp 4 gig for an m3 perfect?

[Knux896]

Which is better (faster, cheaper, more compatibility),
SD or CF?

CF currently has better compatability thanks to chishms current drivers but he is also working to integrate SD. You will probably be able to run older homebrew as well as the newer releases best using a CF though. CF and SD cards generally go for about the same prices so price shouldn't be much of a deciding factor.

[The Hombrew Hunter]

CFs are a BIT cheaper, better compatibility, and SDs are a bit faster.