Well, i won't make my 1 second answer a paragraph long. Which should I take, Jeheira (but miss out on story line) or cerndt (and miss out a kick ass shapeshifter character)?

<font color="gold">I give my vote to Jaheira.
Cause that way, you wont just get a good Druid, you will also get a good fighter. So if you look at it like that, Jaheira is the better NPC.
But if you just want the best Druid, then Cernd must be your choice. He is the better Druid in the game. ;) </font>

Take Jaheira. Cernd can't equip armor, so you always need to keep him in the back of your party, or keep him in werewolf form, but the werewolf form is not very powerfull. YOu are better off with Jaheira. She can wear armor, she can become specialist in weapons, she has a better thaco and health, and she can cast druid spells.