James J. Kilpatrick, in his book Fine Print, says there is no discernible difference between the two: the British prefer towards; American usage calls for toward. Fowler, however, has a more complicated answer. He notes differences in pronunciation and usage (adjective/preposition). Says use as adjective is obsolete and, as preposition, the -s form is the prevailing one, and the other tends to become literary on the one hand and provincial on the other.