First off you need to know what the two are. Mixing is taking pre made sounds cutting em in parts and placing em where you want where as composing is taking sounds from instements and chosing where to place the notes and where other istrements come in. This sounds like mixing isn't a work load but it is you have to fade sounds out chose where to place em what parts to use and still make it sound good instead of randomely placed sounds that don't match as well as having a limit on what you can do. In composing you have fredome to use whatever you want wherever you want but a dissadvantage is you can't use computer generated sounds. This can be avoided if you have programs for both. They both can take a long time in both fourms it can take several years to make a good song.