To make it shorter than OP's reply, from my understanding of the offerings:

- PlanetScale for predictable load. You pick a config (CPU, memory) and if you don't have traffic it sits idle, and if you have traffic it's limited by the config you picked.

- Neon for scalability. You pay for compute hours, so if your traffic is spikey (e.g. concert ticket sales), you don't pay for idle resources during low traffic, and get all the compute you need during high traffic.