The sound test is a good method to understand the sound difference between the 128 kbps and 320 kbps. There is also a difference in the storage capacity between 128 kbps files and 320 kbps files. The 128 kbps files are lesser in size compared to the 320 kbps files. For MP3 collection my preference would be to store the files in 320 kbps because lower the bit rate, the more obvious compression the files will be. Whatever happens to the MP3 format, compression has a long future as every time a potential expansion of definition, storage, and bandwidth occurs it is matched by a set of new compression schemes that attempt to wring more efficiency out of the system .
High quality MP3 files take a lot of storage space; however, with the hard drives becoming cheaper, I prefer to store the MP3 files in high quality. All MP3 files have lossiness, and there are different levels of lossiness, for example 128 kbps needs less space, but will also be lower quality than a larger 320 kbps file, which is still lower quality compared to the 1,411 kbps file, and 1,411 kbps files are lossless. It may be difficult to judge the difference between a lossiness 320 kbps file and a lossless MP3 file, and to understand the differences other factors such as a high quality output headphone or speaker, a trained ear, or a particular kind of music is required, for example a jazz or a classical song.
Audio quality corresponds directly to the file size, and the files encoded with 128 kbps or less carry enough audible compression artifacts not worth keeping, and these low bit rate files sound worthless . The lossless files are flexible and can always be compressed down to a lossier format; however, it is irreversible, and not possible to convert lossy files back to lossless unless those files are ripped. MP3 is a good audio format and would not change in the likely future, and if you do not want convert the files, it is better to rip and store the files.
References
Hacker, S. (2000). MP3: The Definitive Guide (Illustrated ed.). O'Reilly.
Sterne, J. (2012). MP3: The Meaning of a Format (Illustarted ed.). Duke University Press.
