Couple of things...
I'd move the logic to the Change event of the checkbox instead of the Calculate event on the text field, it's more efficient as the Calculate events fire every time something changes in the form.
"(CheckBox0==true)" should be "(CheckBox0==1)", that's what's messing you up - and if you put that script on the Checkbox then you can use "$" instead of the name to reference the current object (equivalent to JavaScript's "this"). Then the script would look like:
form1.#subform[0].CheckBox0::change - (FormCalc, client)
choicetype0="1st choice"
else
choicetype0="2nd choice"
endif
When you test the form in Acrobat you can hit CTRL-J to open the Debugger and it will show you where the error is happening.
You don't need to reference .rawValue in FormCalc.