Speaking from my maritime experience, 2 broken strands of 19 is a better strength percentage than just 1 broken strand of 7 strand...
It is true however that the strands will be more fragile due to smaller cross section, and also, something to think about is that in a 7 strand lay, usually only one is hidden by the 6 outer strands.
Many more (I forget how many exactly) are hidden inside the outer perimeter on 19 strand.
If two of the outer are failed, quite a few more inner ones could be and hidden by the outer layer.
Seems like when in doubt, a pull test might be good insurance, even with new lines.
Only two kinds of parts fail, in my opinion-
Old,
New.
R,
Chris
My point is that if you have one broken strand, you should trashcan the set of lines. Since the wires in a set of 19 strands are so much smaller and therefore easier to damage, then it would seem that the service life is likely less. I am not saying that 19 strand lines aren't strong or nice to use, just that they likely won't last for as many flights.
I do wonder about the effect of two cable lines being twisted together under tension during a series of maneuvers and the friction between them as more maneuvers are flown, and doubt that this wear is commonly encountered in many other cable uses. Would 7 or 19 strand lines take this best? But I also don't think this is normally the most common cause of line damage, so "real world" comparisons might be rare. If you fly on pavement, avoid dragging your lines on the way in/out of the circle, and do your best to avoid having them trampled in the pits. For me, it depends on where I'm flying, what the pits are (grass or paved) and who is around. I prefer to roll my lines up between rounds. I do this by detaching the handle, but leaving the lines connected to the plane.
In contests, some competitors have stated that since they pull tested befor their first flight and did not disconnect their lines, therefore they should not have to pull test again for their second flight. TOTAL BS! Most line damage happens in the pits, with your plane and handle sitting there peacefully.

Steve