I love a good discussion. :)
Comparisons often fail because measurements are inherently biased but there are some contrasts that can be given that might suggest why OpenToonz might be better... more powerful... advantageous.... <insert your descriptive reason for using OpenToonz here>.
There are many things I like about OpenToonz but I think if I had to list only one it would be the exposure sheet. OpenToonz's xsheet gives it an advantage and continuing improvements related to that suggest that things will continue to improve there as well.
I've attempted to use Synfig several times but in each case it didn't gel with me. (I'd have to reinstall and try it out again in order to refresh my memory on the specifics)
With OpenToonz, after an initially steep learning curve (which was resolved mostly by watching the videos of users who were learning OpenToonz themselves) I found OpenToonz to be a great fit and every day I discover it fitting even better. :)
What aspects of Synfig and OpenToonz do you like most?
If you could pick one feature from OpenToonz to use in Synfig what might that be?
And... to be fair... what from Synfig might you like to see more fully developed in OpenToonz?