Flightcrew & EpubCheck

Greetings!

I'm a little confused about the difference between Flightcrew and EpubCheck (4.0.2)

Does Flightcrew use the same validation ... (not sure what the right word is) ... script? engine? procedure?

I've noticed the results are sometimes different. If I were to use one or the other, which provides more complete and accurate results for epub3 — or does it not matter?

Thanks!