The 2 list are seperated because not all people who are registered as a SPUG member want to be in the mailing list. A US piloteer may want to join in this discussion mailing list for sharing information but not interested in SPUG membership. A Singaporean or Malaysian piloteer may like to be a SPUG member but don't like discussion or mailing list. Set the filter to be triggered with every incoming mail. Set the SPUG mailing subscription to non-digest.
Digests are too big and hard to find interested discussions.
So what you will find is that emails sent to the SPUG mailing list will make their way to the desinated folder quietly. You can choose to read them at your own free time, or whenever you feel like it. It is also easier to follow discussions. SPUG organizes meetings. Please refer to this page for more information for future and past meetings.