A IEC contactor has a better overload trip mechanism. It it more sesnsative to single phasing than a NEMA starter.
The IEC contactor is amperage rated, not hP rated like a NEMA. Good practice is to use the next larger size IEC. An IEC starter is not a robust as a NEMA, and I have found them to be more suspectable to overheating. But they are easier to install, smaller, are touch safe (as all IEC products are).
One big disadvantage of the IEC is they are more suspectable to damage. They need to be protected against fault currents by the proper fusing (Bussman has good info on this)
I like IEC starters as they do offer superior overload protections and have more options than NEMA, and you can even get them that will run on Device Net. And you don't have to buy new heaters when the motor is swapped out.
And you can have the best of both worlds, you can use a NEMA contactor with IEC overloads.
Don't foget to use the next larger size IEC contactor.