I have chased after many different types of cushions, each has advantages and disadvantages, and those are especially related to your body type.
I agree with the above advice: unless you have a major back injury or problem, it is better to learn to sit with minimal supports. (This might take awhile, starting with a full buckwheat pillow and gradually reducing the grains to the smallest possible cushion that is still comfortable or tolerable.) I am a heavier person, and I swear by a crescent-moon buck-wheat cushion. Skinnier people swear by a round, smaller cushion. Really skinny people seem to prefer a small/ short rectangular cushion.
I do have a major back injury, so during long retreats I sit with one of these: http://www.nadachair.com/products/
However, I am trading back support for blood circulation on my legs; during a 1-hour sit at least one of my legs is guaranteed to "fall asleep".