I was actually wondering about what would be the best unit to take with the poison banner:
Night goblin archers: cheap and can get nets to better protect the bsb
Goblin archers: slightly more expensive but can get 3 skulkers to protect the bsb
Goblin archers w/ shields: even more expensive but at least they have a 5+ armor and 6+ parry
Orc archers: longer range and better punch in combat
Savage archers: more attacks to take advantage of poison
All in all I think staying cheaper is better, but I may have to try it in the savage orc unit.