The way your radios are set up in the fiddle - sharing the same model - will cause only the last group to show a checked radio if you decide to quote all of the truthy values. A more solid approach will involve giving the individual groups their own model, and set the value as a unique attribute of the radios, such as the id:
$scope.radioMod = 1;
$scope.radioMod2 = 2;
Here is a representation of the new html:
<label data-ng-repeat="choice2 in question2.choices">
<input type="radio" name="response2" data-ng-model="radioMod2" value="{{choice2.id}}"/>
{{choice2.text}}
</label>