It's the Dungeon Master who decides the permanent effect.
The Dungeon Master's Guide is instructions to dungeon masters.
The introduction to the DMGintroduction to the DMG says:
It’s good to be the Dungeon Master! Not only do you get to tell fantastic stories about heroes, villains, monsters, and magic, but you also get to create the world in which these stories live. Whether you’re running a D&D game already or you think it’s something you want to try, these rules are for you.
It is the DM who decides the permanent effects of the potion. A smart DM is looking out for what is most fun for everyone. And you can certainly discuss with the DM what you think the most fun outcome is.
It can be a little confusing because the audience shifts a bit in some of the DMG. In the magic item descriptions, the audience is no longer the DM, it's the character. The potion of healing says "You regain hit points...". Obviously the "you" is the user of the potion, in other words, the creature drinking the potion. However, in the overall book the "you" is the DM.
