Stuffit blows. The .sit compression format is dead.
Back in the day I used StuffIt for Windows and liked it a lot, it's a lot better than the bloat that is WinZip, but alas they've bloated up since then, and .zip has became such a standard that most OSes can handle it out of the box.
Just use OS X's built-in unarchiver, and get UnRarX for rar files, and you're set.