Logical File or OPNQRYF or anyother way ?

Here is a basic question as I am new to AS/400  

I have a physical file which has millions of records, and this physical file 
gets updated quite frequently by various programs in the application. 

I have to develop 2 programs pertaining to an enhancement request.

Program 1 

Needs to read the file selectively based on two fields of the physical file , 
perform calculations on the selected records and update them. There is no 
existing logical file on the physical with these 2 fields as a key
  
Program 2 

Needs to read (only)the file selectively. File is opened in input mode only.

Which is the more efficient method in each case - creating a logical file with 
the 2 fields as key or using an openquery with select on the 2 fields before 
each program is called. Any other efficient way ?
   
Also :- What are the scenarios where a logical file is preferred over an open 
query file and vice-versa ?

Thanks in advance
Malini