What is the difference between the two?
Lemme try and explain this better.
Offline Backup Installer = MOST RECENT/CURRENT VERSION
This version is the most uptodate version that has the most content. sadly tho the gog version of it does not include any signs of support for mods and/or MP. This version also supports the new Paradise Lost DLC
Postal 2 Complete original version in Extras = Old v1409 Before the 1409X patch
This is the version from gog youll want if you want to enjoy any MP from the game. This version also supports mods. This version sadly tho does not support the new Paradise Lost DLC
That should answer the need to know questions ya may have.