For the Dark Angels Terminator boxed set, I had to look at the fact that I already had two squads. So the real choice was between a command squad and a knight squad.
The deciding factor for me was the ability to equip one of the models with a plasma cannon. That was the first model I built and painted. The sergeant, champion and standard bearer all followed from that choice.
I painted the terminators using the same palette as the rest of the army, though I opted not to free hand any Disciples of Caliban insignia at this time.