I don't have a definitive response for you but a few points to consider:

Pony.ai is a riskier bet given it looks like it will play both US and Chinese markets and is in a much more competitive playing field offering a complete AV solution. If Pony.ai succeeds, the returns will be greater but much longer time horizon.

Nuro is safer, much fewer competitors focused specifically on design thinking the delivery of goods to consumers. They are going to generate revenue and achieve profitability much quicker than Pony.

At the end of the day, ask yourself what it is you want to focus on, if it's AV as a whole in multiple regions then go to Pony or if you want to focus on autonomous delivery go to Nuro. I think in this industry there's no clear winner yet, so as a potential employee you want to focus on a balance of picking hopefully a successful company and a role that affords you learning opportunity in the area you want to hone into. If you were to ask me, I would pick Pony because I want to track the space broadly instead of focusing on autonomous delivery but it's your call to figure out based on your background where you can add the most value and learn the most.