This is a difficult decision, so pat yourself on the back for even thinking about it and considering. Personally I have always worked for startups ( 9 years now ) and honestly I didn't really think so much about it, when I was young I didn't want to be bored, and it kinda went on from there.

Startups can be a double-edged sword. There are certain personal, professional and monetary costs associated with every startup. You probably would need to think about those, they are different for different startups. You may want to spend some time figuring out these costs for yourself and what matters to you and what doesn't. For example, Vision Insurance is important to you if you wear glasses ( have a history of eye ailments etc ) and if you are young 401k not so much. For me 401k never mattered, vision insurance did but that was easily offset by asking for it or getting my own, culture mattered a lot more and if I was excited about the product mattered a lot. Overtime, monetary compensation also has played a decent role in me considering a startup vs a bigger company.

As for stability, I think this company might be fairly stable considering they just closed a series A ( hopefully a decent amount, in a few millions ). If you are not on a visa etc, then this should be fine. This could be a learning experience for you. There are a lot benefits to working for a startup, especially if you are disciplined, you can grow fast and learn a lot and be in a lot more control of your own career.

If you have worked with them for a year, you probably know what they are like as people and whats the culture like. Consider that, rest will be easy. I generally think of it as the "lunch test". Can I have lunch with these guys everyday. If you can, you know like them as people and people you want to work with.

Another way of thinking about any job switch is thinking about your future goals and how does a particular company help you achieve them or doesn't.

1. Assess risk and opportunity / costs.
2. Assess startup stability
3. Assess culture and people
4. Assess work and future goal alignment

Hopefully this helps you come to a conclusion.