So the decision should be based on these two things:
1. A Cleric in the WG is superior in the game over CG at this point. The current 2 adv cleric skills offered at the CG are generally less useful than duel and heavy hand. My WG cleric is a beast with all warrior adv skills. Able to dish heavy damage and save mana for healing over dam allowing long easy dungeon runs.
2. The Cleric class/guild is the least developed at this point, but future content will see this balanced. By not being in the CG you prohibit yourself for the future.
Personally, if you enjoy the game enough, I say keep your WGC, train it up to lvl 15/16 (good top range for current content). Then start a CGC so you can enjoy the 'true' cleric adventure.