Both the pinned task strategy and saved task strategy let you re-use a task over and over again.
The difference is that pinned tasks always live in the Master List, whereas saved tasks are not really on any of your lists until you load them.
This means that pinned tasks can be used for Smart Lists (and you can do a whole bunch of stuff with Smart Lists!) but saved tasks can't.
When to use saved tasks
Save tasks are great for tasks that you do more than once but not at a regular interval (use recurring tasks for those) and you want to include some kind of note or checklist (subtasks) in them that makes doing the task easier.
"Order groceries" (the credit card number required for checkout included in the notes)
"Pack for city trip" (including a checklist of what to pack as subtasks)
"Re-order skin care supplies" (list of which items you always get)
"Promote blog post" (checklist of steps to take as subtasks)
These are tasks that you probably do more than once and that include details (subtasks or notes) that save you time when doing the task.
Whenever you need to do this task instead of writing it out you load the task.
When to use pinned tasks
You can pin any task and it will then stay in your Master List for as long as it is pinned and exists.
The useful property about the pinned task is that you can schedule it and it will schedule a copy of the task while keeping the original task in the ML.
So instead of it being a "one off" task it is now an activity that is intended to be done multiple times. But not at a regular interval.
This is useful if you have a day planning workflow and want to keep a list of activity ideas that you can do anytime.
Let's say you have a business and there is a list of marketing activities you can do to help spread the word:
Follow 20 people on twitter
Answer a question on Quora
Create a facebook post
Comment on a blog post in your niche
You could make each of those tasks pinned tasks and then schedule those activities into your day anytime as often as you want. You can also use them in Smart Lists.
You can also find an example of how pinned tasks can be used in this video.
The key difference to saved tasks is that pinned tasks are tasks you can do anytime. Saved tasks are just there to be loaded when you need to do them.
Depending on your workflow it might not make a huge difference which ones you use at the moment, but in the long-term it is best to use each strategy as intended as they will tie in with other strategies in specific ways. Ultimately, Marvin needs to know if a task is something you can technically do right now. And open tasks in the Master List are things Marvin knows you can do but saved tasks are technically not "open tasks" until they get loaded into a day or the Master List.