Never liked Sinclair on Babylon 5, but Sheridan was tolerable.
I LOVED Sinclair. Thought Michael O'Hare bought a real gravitas to the show. Boxleitner's opening narration was amateurish compared to O'Hare's moody first season intro. Also preferred Sinclair's Battle of the Line backstory and eventual fate as Valen. A really distinct, memorable character arc.
I also preferred Sinclair. Michael O'Hare had no baggage as an actor the way Boxleitner did, and he helped usher us into the B5 world without thinking of Tron or Scarecrow & Mrs. King.
Then again, B5 kind of became known for taking typecast actors and giving them something new to do, like Walter Koenig with Bester.