Today SIVR closed at 31.14. SLV closed at 30.46. Both are supposed to represent an ounce of silver. It's been this way consistently. I think investors are assigning SLV a lower value, because there's more perceived counterparty risk.
I agree with the previous poster that PSLV is the best choice among the various ETF's. SLV is very liquid and can be used as a trading vehicle but should not be a long term holding. Physical silver in your possession is the absolute best way to go.
I don't believe SLV is holding all of the metal being claimed and if so there may be contracts against that metal. So I believe SIVR may be safer. However, PSLV would be possibly the better of them all. I hold SIVR and PSLV. I will probably keep both but I favor PSLV. Finally, you need to purchase and hold for long term physical silver its the real deal. You buy and ounce and it stays an ounce without counter party risks. By and hold physical! Silver is being depleted via commercial use in appliances, phones, computers and little of it is being reclaimed.