The mixed mode is more intelligent, searching buffers, MRU, and the general file list. It gives you more of what you want, and less of what you don't :)
We are purposefully including an older version of snipMate (0.84) from mikefarmer's repo so that it works with the long list of snippets available from the scrooloose/snippets collection. The current mainline snipMate seems to be no longer compatible with those snippets, but the scrooloose collection is still the best one to use.