Both and are the same except for the fact that if you want your template/code to be according to be HTML compliant and follow the best practices. Use data-ng-app.
This is what the official documentation quotes:
"Best Practice: Prefer using the dash-delimited format (e.g. ng-bind for ngBind). If you want to use an HTML validating tool, you can instead use the data-prefixed version (e.g. data-ng-bind for ngBind). The other forms shown above are accepted for legacy reasons but we advise you to avoid them."
Hope that answers your question.
Cheers!