I don't think the OP said "rebuilding". I took the OP to mean that at startup #1 they have an idea and a plan, and they need the OP to help execute on it. To

I don't think the OP said "rebuilding". I took the OP to mean that at startup #1 they have an idea and a plan, and they need the OP to help execute on it. To the OP > IMO you will probably get a good rounded experience from #1, but it is the most risky. Also, if you cap yourself at 1 year and you're building from scratch, you'll probably leave just as things start to get interesting at startup #1. Right as they start to get traction etc. I would go for #2. It sounds like the best place to get 1 year of valuable experience, honing skills that you already have on live products. I think that's the best use of a year, rather than trying to get experience on a bunch of technologies you might never use again a la #3.