A mix of junior and senior folks?
If possible. Give a leg up to the little hu.
Including different theoretical perspectives?
Yes.
Selecting those papers that actually spoke to your description of the session (some of my submissions are very narrow in their scope)?
Not so much. If you can put together a killer session using papers that wander a bit from your original vision, or a lousy session that fits the original bill, go with the former.
Then remind yourself if you are on the fence between two papers, it probably won't affect the overall quality of the session.
airball