Since the sync detector on your R-75 is presumably working better with
the Kiwa sync' mod

Nothing 'presumably' about it, heheh! It improves the synch drastically!

do you prefer to listen to an AM signal using the
sync' or do you still use ECSS, like many other R-75 owners do?

Most often, if the signal is stable, I'll not use either. When needed though, I
do use synch over ECSS. To my ears, the fidelity of a synch'ed signal is nicer
than in ECSS, and with the 'fixes' in the circuit involving the AGC and so on,
it works quite nicely. But of course ECSS is a tried-and-true, dynamite tool
for DXing, and I'll use it at times if the synch can't lock on a really
weak/fading signal (the R75's synch, even when fixed with mods, can only lock
so far) so I won't say I use the synch exclusively. But more often than not the
synch is engaged for tough signals.
It nice to have a working synch detector and very stable SSB features, and have
the choice whenever you need to use one or the other.
Linus