> Don't know numbers off hand but we had a <4% rate ARM because it was the lowest rate and we liked the flexibility of paying it off asap (or if something happened, paying it off over 30 years) versus a 15-year fixed.
you can pay off a 15-year mortgage in less than 15 years - the lien will be released, etc whenever the principal is paid. Everything depends on the specifics of your contract of course but it would be extremely unusual to have a mortgage where this is not allowed.
fixed vs ARM purely depends on how the interest rate is determined, that's it. In principle ARM should be a bit lower than a fixed rate financed at the same time - but if the prime rate goes up then your rate goes up too, where with a fixed it's locked-in forever. Fixed will have a higher interest rate because someone has to assume that risk of an increase in the prime rate, where with an ARM that someone is you.
Taking an ARM vs a fixed is a bet on whether rates are going to stay the same or decrease, vs increase. And boy if you thought 2020-2021 rates weren't going to increase at some point in the future, uh... it's not every day you have a once-in-a-century pandemic that nukes the economy and drives demand for money almost to zero.
I'm not quite sure what you're saying about balloon payments either, the balloon payment happens at the end of a balloon mortgage, with the intention that values will have gone up so you can refinance at that point - but of course, what if they don't?