Am looking for advantages of IEC Motor Control Center VS. NEMA (& Vice Versa).

IEC conforms to international laws and standards, NEMA to ones in north America.

I suggest to use same standard for MCC (protection devices from inside), cables, supports for cables, lugs and connectors, frame dimensions and rating voltage. If you mixt them, will be more expensive (a lot of work, incompatibility, adjustments etc.). Practically is the same if you take and apply the entire same system.

IEC advantage = small form factor,Lower cost
IEC disadvantage = Cannot dissipate heat as rapidly as NEMA

A very good technical reference for this is NEMA ICS 2.4-2003, "NEMA and IEC Devices for Motor Service—A Guide for Understanding the Differences." You should be able to download this from the NEMA website at www.nema.org.
By the way, we are one MCC supplier who uses IEC components in their NEMA MCC design. We use IEC auxiliary relays for control purposes quite often. We also have a compact size 1 starter that uses an IEC contactor that is NEMA rated. So yes, to an extent, you can mix things up. But the overall standard for the entire assembly must be one or the other.
Then there is the issue of listed equipment. If your project requirements include UL listing, the assembly, be it IEC or NEMA, will need to comply with UL 845. UL 845 refers to UL 508, Industrial Control Equipment as a normative standard. UL 508, upon cursory review appears to be organized around NEMA standards.