That's a great question, thank you! It made me dick (edit: standing by my mistake!) a (tiny) bit deeper. I took a different perspective and the tldr is: Do you want to kill specifics? I.e. local plants, animals, water poisoning, etc - then mercury is the winner!

If you're after killing via global temperature variation then the car is.... Well... Killing it.

But on a serious note: both are bad but depending on how your local trash is handled those small bulbs could actually have an impact, most likely via the water chain.

If those are the two options I had I would just store them like OP. But then again where I live most shops take those back to recycle them properly.

Thanks again for the question, I had a fun few minutes!