Here's the difference, from their books, the difference is massive, the philosophy resides on opposing poles
PADI SELF RELIANT
Self-Reliant Diver
Distinctive Specialty Course Instructor Guide
General Purpose
The purpose of the Self-Reliant Diver specialty course is to recognize and
accept the role of the buddy system and its contributions to diver safety
while identifying and developing self-reliance and independence while
diving. There are two reasons for an experienced diver to take the
Self-Reliant diver course:
•To develop the skills of planning and carrying out dives without a partner when preferred or necessary.
•To sharpen skills of diving self-reliance, making the diver a stronger partner in a dive pair or team.
This course covers when diving alone may be applicable, and the need to compensate for those situations, including dive planning, life support system readiness, adaptive training, equipment and responsibility.
This course is an introduction to self-reliant diving that helps student divers develop the skills, knowledge and techniques necessary to rely on themselves first, whether or not they are diving with a partner, including:
•The value and application of the buddy system.
•The philosophy of, and motivation for, diving without a partner.
•Potential risks of diving alone, and how to manage those risks.
•The value of equipment redundancy and what back-up equipment is needed.
•Dive planning and gas management.
SDI SOLO