I've heard it said that statistically over the life of a 30 year term an ARM ends up costing less.
I have nothing to back that up with.
Under the correct conditions I can see that being true.
In other words, timing.
There are some people here with better knowledge of the subject than me so till they chime in I have a pretty chart for you.
Wish I had one for the coming 30 years as that would be more helpful.
Personally I don't see money getting any cheaper or not enough to make much of a diff on your monthly nut. Best bet is it only goes up from here which ='s not good for an ARM.