This decision is all about what you feel is needed in your show/act. Are you looking for a new opener? If so, go for SD3. Are you looking for something to enhance your magic? If so, go for Pure Smoke. It's difficult because they're two completely different tricks with different purposes, so it all comes down to what you feel your show/act needs.
But if I had a gun pointing to my head saying i can only have one out of the two, I would say Pure Smoke.
If you plan to go for Pure Smoke, I would recommend picking up Gregory Wilson's DVD "Pure Smoke Masterclass" (or something like that) at Ellusionist. On that DVD, you will find tons of great effects using the Pure Smoke device, some of which were performed by Dynamo on his TV show.
Hope that helps.