Quote from HoustonMark:
Price data accuracy and data lag are totally different concepts....however, a lag of some number of milliseconds at report time doesn't affect my trading to respond to your comment....
"Price data accuracy" is not what is important it is "Price data real-time accuracy" what counts...in this respects an accurate real-time Best-Bid and Best-Ask is more valuable to have than a Last-Trade-print (which is already history {trade is done in the past: but Bid/Ask is the next thing in future}).
Streaming-date providers like eSignal, gives you Last-trade-prints in history...as OldTrader says: this history is in milli-secs when the market is not moving. but during news events we are talking about 10 or more FULL seconds (I even have seen a nearly one minute lag, compared to IB {I have both running side by side}).
Snapshot-data providers like IB (and X-trader) gives you the CURRENT best-bid, best-ask, and last-Trade..each and every 1/3 second without any Lag. just every 1/3 second you get the current Market situation, which is invaluable during moments when it REALLY counts: during times that market moves highly volatile.
I think each trader need both providers, but for different things. Charting and TA you do with a Streaming-provider. and trading you do with a Snapshot-provider......and how knows..one day some software will integrate best of both worlds ;-)