XF 1.2 Download the upgrade package only OR the whole package ?


I were going to download the latest Xenforo 1.5 and got the question if I wanted to download the whole package or the upgrade package only.

My installation of Xenforo is at version 1.2.1. The logical alternative is to only download the upgrade files but
since I have version 1.2.1 up I might have to download and install a FULL installation ?

Do I ?