There is a lot of confusion over these two clematis. The nursery trade has confused them and the confusion is residual. Some plant tags/ nurseries still call terniflora by the name paniculata.

What I consider Sweet Autumn is terniflora. They start blooming in August and September in SW Ohio. There is one called Summer Snow or Paul Farges that just finished blooming here. It looks more like terniflora than does paniculata. Look at the the pictures at Clematis on the Web. There are good pictures of each.

Hope this helps. This is a good case for using botanical names.