I would think it all depends on the situation. If you've got to move, double tap the SWD. If you've got procs waiting, wait for the internal. I feel like my dps has been the same in either situation though.
Since the difference between the cooldown and a proc based SWD is only 2 secs i would think the time window in which you can let SWD take advantage of that extra orb is rather tight, calculate into this various lag enhancing annoyances and other crap and you could end up hitting SWD just a bit too soon resulting in still triggering that cooldown which then results in 14 secs of no orb generation through SWD.