by default Cyber uses AMEX [through ABN]this is terrible.
Use Island only. There is no comparison. You will get filled in 1 or 2 seconds and little slippage.
However, it is tough to daytrade QQQ, you probably want to catch longer swings, even purely intraday. Forget about scalping it, it is very difficult because QQQ follows so closely the futures. You would be better of trading the futures themselves [if enough capitalized of course].
QQQ, Island or Instinet only for trading [Island is best].
vpiNeo