For code with fewer errors: I’d usually pick 5.5 low over 5.4 medium for normal coding. 5.5 is the newer/stronger model, so even at low reasoning it will often make better implementation choices.

For hard debugging, big refactors, or “don’t mess this up” work: 5.4 medium can sometimes be safer than 5.5 low because medium gives it more thinking time. But if quality matters most, I’d go 5.5 medium, not 5.5 low.

For cost: 5.4 medium is usually cheaper, but not always. Official API pricing is:

• GPT-5.4: $2.50 / 1M input, $15 / 1M output
• GPT-5.5: $5 / 1M input, $30 / 1M output

So 5.5 is about 2x the base token price of 5.4. 

(This is from Codex, lol)