If you are not a native speaker English, like myself, these are difficult issues. According to Burchfield’s third edition of Fowler’s Modern English Usage (1996), the one is characteristic of American English and the other of British English. (Fowler himself hadn’t commented on this particular difference at the time of the first edition.)
Because my linguistic model is British English, I suggested towards for the title of a book I was editing jointly with Dieter Stein in the early 1990s, and Dieter happily accepted it. The publisher, however, silently changed towards into toward, and it took a lot of work to convince them why it had to be towards. As the image here illustrates, we eventually won our case, and Towards a Standard English it became!
In the stylesheet they use today, Mouton de Gruyter, our publisher, allows for both British and American usage, as long as authors are consistent.