I don't think I could make a recommendation without knowing more because it depends somewhat on degree...
bipolarBipolar disorder
Bipolar disorder is rather loosely used. Both of those medications have been used effectively, and if your doctor chooses either it will depend how you tolerate the dose and the side effects.
My suggestion is: if you're not severe bipolar (i.e. mania followed by depression and viceversa) don't take any mood stabilizer. Try to wait another 3 months, and look what happens.
Lastly, lithium is not the best. If you can, chose something like Depakote....or, at least, something that you'll not take for the rest of your life (i.e. Lithium).
Good luck.