I would recommend using Delta. Delta stores data as parquet files so you still get a lot of the benefits of parquet with Delta. Even though you don't need to merge data, I would assume you will still want to take advantage of the update/delete functionality of delta. Plus delta will offer better optimization techniques to ensure that your data can be queries efficiently (file pruning, z ordering etc.).

The conversion between delta and parquet is easy so you can always test out both and see which you prefer.