We’ve noticed that sometimes the real-time flight data returned with FlightXML3 API is incorrect (we verify this with the airport).
If I use FlightXML2, would I get different results or is FlightXML3 really just a new API over existing data?
Examples would make things easier to explain. FlightXML2 and FlightXML3 pull from the same data source.
If you mean examples of data being incorrect/missing, see most of my previous posts 
As an example, yesterday’s scheduled flights (7th Feb 2018) for Southend (SEN) include (actual data from ground staff at Southend in brackets) :
Arrivals:
REA6231 09:30 ( touchdown 09:21 on stand 3 at 09:25)
BEE6025 13:20 (touchdown 12:49 on stand 3 at 12:53)
BEE6171 13:30 ( touchdown 13:15 on stand 8 at 13:20)
Departures:
REA6230 06:35 (pushed back from stand 3 at 06:50 airborne at 07:00)
BEE5251 06:40 (pushed back from stand 8 at 06:51 airborne at 07:08)
REA6024 10:05 (pushed back from stand 3 at 09:57 airborne at 10:06)
According to the AirportBoards API, these flights haven’t yet arrived or departed (even though it is 24 hours later).
Other flights for this airport have arrived and departed according to the AirportBoards feed.
But some of those flights show up on FlightXML2 and not FlightXML3 Beta or vice versa?
I haven’t the time (or budget) to investigate if FlightXML2 does give the correct data.
If the underlying cause of all our missing flights is that we shouldn’t be using FlightXML3 Beta for live projects, I can relay this to the project manager and see if there is the budget to re-write our import component to use FlightXML2,
Can you @cbw confirm this should be our approach?
The two APIs source from the same data. It looks like the missing flight from your other posts and the missing values here may share a root cause. It’s likely that as that issue is resolved these flights should appear as you expect.
At this time REA6231 and BEE6171 are showing block times (if using include_ex_data = true).