Users should almost always choose Delta over parquet. Keep in mind that delta is a storage format that sits on top of parquet so the performance of writing to both formats is similar. However, reading data and transforming data with delta is almost always more performant than Apache Parquet. Additionally, Delta has all the same benefits of parquet and more.
At times I have seen people chose parquet over delta if they are dumping data to external systems that do not support delta.