I can't answer your question in full, having not heard the SA6, but I have some questions for you that will help others assist your choice. Both will isolate fairly well, and my 780's don't leak much sound at all (they leak less than my Edition 9).
What kind of music do you listen to? (rock, electronic and new age are best with HFI-780, jazz and acoustic is still pretty good, classical might not be the best with either)
Do you prefer the soundstage of full sized cans that interact with your outer ear? (the HFI780 are wide but not deep, unless you re-cable)
How much portable and how much home listening will you do? (IEM are easier portable, with DAP in a shirt pocket)
Will you be laying down with them or falling asleep with them? (IEM that go in deep like SA6 win)
Will you be listening for extended periods of time? (you may get itchy ear canals like me after a while, or you may find the 780 clamp too much but can be stretched out)
Have you tried IEM before, so you know if you are comfortable with something deep in your ears?
Do you mind pulling out an IEM every time someone tries to talk to you, or is lifting a full size can off your ear better? (780 win here, because my wife can lift them off after the 3rd time she tries to get my attention)
Are you okay with not being able to tune the treble on the 780, which can be bright with some sources or amps? (choosing an amp to control the bass and treble is one way, or stuffing the inside of the 780 with dampening material is another, re-cabling is another)
Do you like to burn in headphones for 300-500 straight hours before you like the sound enough to keep them? (then the HFI780 are not for you unless you get the ALO modded ones, then they sound good in 50 hours and are fully burned-in by 200).
Do you like to be totally immersed in the music and be mesmerized by the music? (you want to get up and get a drink, but can't bring yourself to un-plug from your home rig to go get it, then the HFI780 are for you).