I would take Branwen over Yeslick hands down. Firstly, she can be acquired relatively easily; if you have enough coin. Or a high pick-pocket skill, but I digress.
Yeslick's Dispel Magic special ability is better than Branwen's spiritual hammer, but doesn't get rid of the fact that he gets fewer cleric spells, being that he's a multi class.
I'll be taking Branwen