From a performance perspective which is better at sorting and what dateTime functionality is lost by not using dateTime as the type.
If you treat datetime values as strings then you can only do lexicographical sorting. With the date time indices you can do finer-detailed sorting based on year, month, day, and/or hour (with @index(hour), you generate all four indices).
As mentioned in the datetime indices docs, it depends on your use case which type of index you choose to use.