In a new job, do you prefer to join a newly spun up team, or an existing one?

Existing when I join new company. More opportunities to learn from SMEs and team is already proven to be value added.

One risk of joining a new team is that, since you’re still in that growth period, they might hire people you don’t vibe with. Conversely you’ll probably have the most visibility / incumbency advantage when it comes to matters of company politics.