In general, mallet choice depends on the sounds you are looking to achieve. Most mallets will come in a series ranging from soft to hard. In general, harder mallets are used in higher octaves, softer mallets in lower octaves.
Yarn mallets are perfect for marimba playing. Depending on the sound you are striving for and range you are playing in will determine what type of hardness you need. Yarn mallets also work nice for multi percussion and cymbal work.
Cord mallets are the standard for most vibraphone mallets but can also be used on the marimba. Like yarn mallets, they range from very soft to extremely hard.
The world of unwound mallets (mallets without cord or yarn wrapping) are meant for any instrument from the xylophone, glockenspiel, to woodblocks, temple blocks, trash can, and the kitchen sink. They can range from very soft rubber, to the brightest of brass mallets.
"Unwound mallets consist of various oval shaped rubber, wood, metal, and plastic heads to supply multiple timbres and dynamic levels. These are designed for response, clarity, control and sensitivity." -Republished with permission from MikeBalter.com