In summary, SSB is, from what I can see, designed for non-public sharing of Twitter-like content between friends, and your data only traverses to people who follow you.
In contrast, Aether is explicitly designed to be a public space where you talk to people you don't know. It communicates everyone's information to everybody. To make that possible, Aether only accepts and transmits text (you can still link to stuff), while SSB can transmit other formats directly in the protocol itself.
(I might be horribly wrong: I'm not a SSB expert. Please correct me if so.)
In summary, SSB is, from what I can see, designed for non-public sharing of Twitter-like content between friends, and your data only traverses to people who follow you.
In contrast, Aether is explicitly designed to be a public space where you talk to people you don't know. It communicates everyone's information to everybody. To make that possible, Aether only accepts and transmits text (you can still link to stuff), while SSB can transmit other formats directly in the protocol itself.
(I might be horribly wrong: I'm not a SSB expert. Please correct me if so.)