How extensive is their questionnaire library? There's no reason to 're-create the wheel' if it's not absolutely necessary. Can an existing questionnaire template be used as a baseline and then customized to fit the specific needs of your questionnaire?
What flexibility is built-in if you have a unique need and want to create a questionnaire completely from scratch?
What about advanced question types, and logic such as branching and scoring? Will the questionnaire solution allow for that? If so, how easy is it use.
Are there themes to choose from to customize the look and feel of the survey?
QuestionPro's answer to these questions and more, is YES!