Since our session 0 we've included the Potion Miscibility variant rule but never used it 'til now.
The party (lvl 5 now) has hoarded quite a few potions - all common or uncommon level - and only used healing potions. At the end of the last session the alchemist decided they would mix a few together, after a few tries of varying success they rolled 100 on mixing a Potion of Growth and a Potion of Fire Breath.
The rules for rolling 100 on mixing potions is:
Only one potion works, but its effect is permanent. Choose the simplest effect to make permanent, or the one that seems the most fun. For example, a potion of healing might increase the drinker’s hit point maximum by 4, or oil of etherealness might permanently trap the user in the Ethereal Plane. At your discretion, an appropriate spell, such as dispel magic or remove curse, might end this lasting effect.
I ended the session with a dramatic description of their swirling potion - however I'm not really sure how to proceed. The players had a win and I want them to be rewarded for that - I'm just not sure which effect is most balanced. Also, unsure whether "Choose the simplest effect" means DM chooses or the player chooses - may not even be up to me- I've asked that in a separate question. Even if it is the player's choice, an idea of the imbalance this could cause would be useful to discuss with them/prepare myself for.
Permanent Growth: A player gets permanently enlarged - gains a size (medium->Large or small->medium), +1d4 damage to any weapon they were holding at the time and advantage on strength checks and saving throws.
Permanent Fire Breath: A player permanently gets a breath weapon that
A bonus action to exhale fire at a target within 30 feet of you. The target must make a DC 13 Dexterity saving throw, taking 4d6 fire damage on a failed save, or half as much damage on a successful one. The effect ends after you exhale the fire three times or when 1 hour has passed.
I considered making the breath weapon have no time limit, but still only 3 uses. However, that feels like it cheapens the win. I don't want to do that to my players.
As a comparison, they're both like getting a second level spell free...permanently. For fire breath the save is likely lower, but its damage is better than the 2nd level dragon's breath spell - though that is an area effect and fire breath is a single target. Growth is a straight copy of the 2nd level Enlarge without the option to reduce.
Which of these is the most balanced option?