At the recent Society of Garden Designers Awards I was asked by a non-member why they should consider joining the society, and that as a member of the Landscape Institute they had a much more rigorous training and so the LI credentials were better.
Not being great at thinking on my feet, especially after absorbing hospitality, I gave my defence that the Society promotes excellence in garden design and that this is something worth striving for. Thinking about it later though I believe the difference between Garden designers and Landscape Architects is that GD's form a much closer relationship with their clients and develop a much more personal brief.
Usually with more human idiosyncrasies, which to my mind is the thing that makes garden spaces more interesting and better connected/related to the people using them. My most successful gardens have been those where the clients are closely involved with every stage of the process. My role is still to come up with a scheme they weren't expecting, but are delighted with, and that has their stamp on it.