Not sure and tbh ill call draw.
Due to kakuzu's iron skin he can protect himself from any weapon drenched in poison from sasori. Im certain though that iron sand would get past his iron skin.
Secondly no matter how many hearts you have if those hearts are in kakuzu and not on the outside, if kakuzu was to get poisoned he wiould die no matter how many hearts he has. If those hearts are on the outside, tbh they would get fodderized. Those 5 hearts cannot take on the many puppets sasori has.
Tbh after thinking about this im leaning towards sasori.