Ask HN: What features and aspects to look when selecting a startup to work for?

I have applied in 2 startups both in 3D printing domain. One consist of young and energetic graduates working on some projects related to it and also involved in selling a paid on too. But on the other hand the 2nd one consist of experienced entrepreneurs working on a single project related to 3D Scanning and printing, its unpaid. Please help on how to select, what main points should i look.