The bigger and more stable environment will be better for the anemone for sure. Just mae sure to match the params as close as possible and you will probably want to move the anemone with whatever it is attached to.
Of course anemones like well established tanks best, so if it is small and the 20 is well established you may wish to wait until the larger tank is more established. But if the 20 is recently setup then there is not much difference.