It depends on what kind of tea you made. Black tea has more caffeine then green tea does. Soda is really bad for you in so many ways. I cant believe this information is not out there. Water is the BEST thing to drink to rehydrate... Since the main ingredient in tea is water... it is safe to say it is more hydrating then soda. Most soda's main ingredient is high fructose corn syrup which is found to be a federally approved TOXIN. High fructose corn syrup can and will cause you to become diabetic.... and the acid in the soda will rot your teeth.
Tea has more caffeine than soda or even coffee. You can purchase decaffinated green tea however, I enjoy black tea for the good properties it has and sleepy time tea when I am restless. The only unfortunate thing about tea is it has a very drying effect in the body.
As Asobol said, herbal tea is caffeine free. It also comes in a wide variety of flavors. My personal favorites are Peppermint(or any mint), Blueberry or Peach(so very sweet), and Chamomile(helps me relax at night). Tea is superior to soda when you wish to rehydrate, but ultimately plain water is your best bet for that.