The quick and simple file is that the logical file will already have the index built and therefore has a speed advantage over OPNQRYF which has to build the index before it can be used. The down side to the LF is that it takes up space on the disk.
The primary advantage of the OPNQRYF over logical files is the flexibility of being able to create indexes on the fly as and when needed.
The decision of which to use usually depends on the number of records in the physical and the frequency of use of the index.
Alternatively, embedded SQL is being used more and more for ad-hoc file access.
All the best
The size of a LF is not a downside. The size is negligible. The only downside is that a LF with keys includes an index that usually needs to be updated when records are added to or deleted from the underlying PF. Ongoing index maintenance can become a performance issue when the number of LFs is large and the volume of adds/deletes is also large.
OPNQRYF can replace a LF by having the index built all at once, only when actually requested. If an index is rarely needed, it might be less intrusive over a period of time to build the entire index than to continually be inserting and removing keys from an index.
One distinct advantage of OPNQRYF is that it is the only way to create an index with a key that has subfields from different PFs.