With statehood they give up the option, equally available right now, of voting to become an independent country, with all the perks that go with being a separate nation. No doubt the UN would be glad to admit them as a full member and help them out with all sorts of goodies. They will also lose some concessions which they enjoy as a territory but will not as a state, tax breaks and the like.
If they vote for statehood, they are "going all in" with the USA. They have not wanted to, up until now; so I would wonder, why now? If I were a PR citizen I would vote for the status quo. But I'm not.
Politically, Islanders have a viable two party system. They are not like mainland welfare clients. It would be a mistake to write them off as automatic welfare democrats.
That's my off-the-cuff take, others may differ.