I'm surprised skill development hasn't come up (or if it has, that it's not more prominent - I might have missed it in the scanning.)
This might be your first startup, but it likely won't be your last - and the skills you get from your first startup will affect the jobs you'll be able to land and the projects you'll take on in the future. So the most important factor is the details of what you'll be doing and how much doing it will make you a better programmer.
If you optimize for the startup's overall potential but spend your time there doing stuff you think is trivial, you're going to be really unhappy if the startup doesn't work out. If you optimize for challenging, enjoyable work that turns you into a programming badass, if the startup doesn't work out, you're still going to be a badass.
This might be your first startup, but it likely won't be your last - and the skills you get from your first startup will affect the jobs you'll be able to land and the projects you'll take on in the future. So the most important factor is the details of what you'll be doing and how much doing it will make you a better programmer.
If you optimize for the startup's overall potential but spend your time there doing stuff you think is trivial, you're going to be really unhappy if the startup doesn't work out. If you optimize for challenging, enjoyable work that turns you into a programming badass, if the startup doesn't work out, you're still going to be a badass.