I would go with what caselec said, I looked at the site (look at contractor corner)
and they say,
The perception that steel fittings will outperform similar type fittings manufactured from die cast zinc is untrue. In fact, just the opposite may be true.
But they do not seem to back this up, they tell you that steel and diecast fittings pass the same test at UL.
Do your own test, couple two 10' EMTs together first with diecast and then with steel.
Hold this straight out from the end and shake well.
Diecast will split, the steel will hold out.