Physical Silver ETC: iShares vs WisdomTree

Freetrade used to only have WisdomTree PHSP at an ongoing charge of 0.49%.
I recently noticed that iShares SSLN was added as well (0.20%).

The iShares version seems to have 10 times the daily volume at less than half the ongoing charge.
Can I assume that there’s a smaller bid-ask spread on SSLN as well because of the higher volume? I couldn’t find any details on this.

Is there any reason for buying/selling PHSP over SSLN?