Is there a difference in functionality between the SC15 and SC21?

There is no difference in functionality – both cables allow connection to MFi-certified microphones, such as the AI-Micro or Wireless GO II, to iOS devices that have a Lightning input. The SC15 cable has a right-angled connector on one end and a straight connector on the other, whereas both ends of the SC21 cable are straight. Additionally, the SC15 has a junction box for cable management.