Make your flag, in it's default state. Definitely use Ripples for the animation - nothing else Strata has at the moment would work better for a flag animation. When you create your ripple, make sure that the wavelength is very long, and the frequency very low (well, depending on the effect you want).

Apply the ripples in a planar fasion and use the texture positioning to move the ripple off of the flag .. but making sure that it extends to cover the bounds of the flag. Voila. Ur done. Apply that in any fasion on top of any textures that you have on the flag and you're done (Make sure to do a BurnUV on the flag textures). Then, to change the magnitude, 'wind' speed, etc. just change the ripple to get the effect you want. Add multiple ripples, and a ripple that has a hugh displacement to get multiple effects that will mimick the random movement of a flag in the wind. If you want to make it look like the flag 'falls' when the 'wind' dies down.... that's when you'd be using the bezier animation.... or if you're really bold...deformation lattices - a very powerful tool for anything.