The "Upgrade" is 6.5, the latest version of the feature set.
The "Maintenance" is 6.5.1, which is the same but with bug fixes included for little bits and bobs here and there.
Either way, as already stated, you are best off having the latest version.
"I decided to call my music 'organized sound' and myself, not a musician but 'a worker in rhythms, frequencies, and intensities'”
Edgard Varèse 1962