I have the original Defiant (not the Soul) and I also have the 919C.  1 of my teammates does have both the 919C and the Soul.  Drilled similar, his Soul goes a little bit longer but has a stronger move in the back.  It also appears that the Soul is more versatile too across the patterns we all see each week.

My own experience with the 919C is that it tends to be a lot more condition specific than I like.  It hits hard and provides a good look on some medium-heavy patterns when the backends are cleaner.  Now comparing the original Defiant and 919C, hands down the Defiant is better.  I find that the 919C will not handle heavier patterns as well even though it was marketed as that when it came out.  Because the 919C is much more condition specific, I'm planning to sell mine and potentially get a Soul or possibly a Storm Lucid or RG Critical Theory in the near future.

All out honesty, you're better off going with the Soul.