Browsers have slightly different behavior. Some of those differences are documented.
IE 6
The onChange script is executed when the field is left, i.e. you click somewhere else. If the script is to be executed is submit(); and you want the script to be executed when the button is clicked, you may want to use onClick instead.
The onClick script is executed when the radio button is clicked.
Firefox 1.5
The onChange script is executed when the radio button is clicked. If the script is to be executed is submit(); and IE browses will also be used, you may want to use onClick instead.
The onClick script is executed when the radio button is clicked.