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
if you get info on a file it will tell you what kind it is, in this case Apple lossless. The difference between importing a song this way and regular AAC or MP3 is size. Lossless is roughly 10MB per minute, AAC and MP3 is about 1MB per minute, so a 4 minute song lossless is about 40MB compared to 4MB.
By importing a song as lossless, you don't lose any of the song, whereas AAC and MP3 throw out the stuff you don't really need to hear and enjoy the song, thus the reason it is alot smaller in size. You can increase the bit rate that you import your songs in using AAC or MP3, making them larger, but not by much.
This will be help you..

Use the Apple Lossless encoder for songs that you intend to burn onto audio CDs, as well as for playing on iPods. The files are just small enough (about 60–70 percent of the size of the AIFF versions) that they don’t hiccup on playback. you can download here.