They hate each other enough and are savage enough that I would imagine they probably go for civilians first. After all, Iran knows its only a matter of time before the USA is dragged in to "democratize" their government and turn their people into jewish slaves, while Isntrael knows that eventually the USA wont be there to save them since theyve poisoned it to its deathbed. So both are likely trying to ensure that no one is left to re-populate after things go fully hot.