It depends on what sort of data you are expecting.
Expanding fields are great for compact forms where the user isn't likely to need multiple lines all of the time but you still want to provide the option for longer inputs. Generally these are used for the message input in chat interfaces where comments tend to be short, one-line statements but can be entire paragraphs if required.
Text areas on the other hand are better used when the required input is generally longer. The additional space indicates to the user that they can provide more information and encourages longer responses. Scrollable text areas are more familiar for web users so should be prefered unless you are trying to save space in your design or discourage longer inputs without a hard character limit.