CodeX:
You understand it.
Human reaction time with respect to a Go/NoGo stimulus in younger people (20's) is about 200 milliseconds if they are poised to respond (finger on the mouse and waiting to act).
Depending where you are in relation to your broker or the data provider there is an internet transmission latency (delay) of 10 to 100 milliseconds(ms) . Add this to the 200ms or more response time and there will be about 220ms to 400ms round-trip response time. There is also an additional delay while your broker checks your margin level or cash balance to determine whether to allow a trade.
So - You are correct, either feed will work. However, if you have developed an approach that is dependent on entering at a specific point over or under a price you could miss that point if it fleetingly occurred within IB's sampling period and a trade replaced it. However, and this is very subjective, if you miss it there is always another trade just a bit ahead.
Just to confuse things a bit more, if the exchange sends a price\bid\ask to IB .00001 ms prior to IB's sample, you'll get that data without any real delay other than internet latency.
I suggest you use the ET search feature to find terms like latency, ping time, colocation and co-location. Just remember that many commenters seem to pull answers out of the air.
Jack