My bet is you're going to get a lot of answers that talk about strength and rigidity. I don't know how to evaluate either of those factors. I can't remember ever seeing a step through bicycle that had a broken frame so I'm going to guess they're strong enough. Rigidity is something you might be able to get a feel for with a test ride. If you don't feel anything (my bet) I'd say that's rigid enough.
I'd decide by evaluating the rest of the bicycle. What does the riding position feel like? Does everything work crisply? How do the seat and handlebars feel? Weight matters because you'll have to lift it up stairs, loading it into your car and the like. Even the color matters because, if you LOVE it, you'll feel better every time that you ride it.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.