Best download encoder on iTunes

Can some one answer a question I have had for quite some time asI am having a hard time grasping encoding file I should be using.
 I understand that most iTunes downloads are now in the Purchased AAC Encoder which is an M4a format. What is the difference in using the Apple Lossless file which is also an M4a format? Which is best for a good quality stereo sound system: the AAC Encoder or the Apple Lossless? Thanks
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