This varies from broker to broker, but on IB the Silver margin requirements (large contract) for the eCBOT are 20% higher than for the NYMEX (Comex).
Currently for ZI (5000 oz) on eCBOT:
Intraday Initial -- 8,100
Overnight Maintenance -- 6,000
Currently for SI (5000 oz) on NYMEX:
Intraday Initial -- 6,750
Overnight Maintenance -- 5,000
And as far as liquidity they're not even in the same ballpark, the SI blows away the ZI by many multiples.