If you are in a commercial building, I would suggest MC or BX with isolated ground receptacles. Lots of motors in commercial buildings: air conditioners, elevators, pumps etc. which can cause emf reflections in your circuits.
"THHN" is nothing more than a name or identifier. It means the copper wire is covered by a sheathe of thermoplastic insulation which has certain resistance to temperature, chemicals, abrasion, whatever. This is the red, black, white or green color of the wire. The important thing here is that this designation is meaningless - it only refers to the jacket NOT the wire or any properties thereto. Other designations for wiring is THWN, RHW, RH, XHHW, and dozens of others. Again, not important to you - the copper underneath is all the same for a given size.
BX is a designation for a set of copper wires (covered with THHN RHW RH XHHW whatever plastic) inside of flexible aluminum armor. This whole package is referred to as a "raceway".
Don't sweat it too much. Raceways of BX, MC or Romex each has the exact same copper wires in it. The only difference is that Romex and MC doesn't pick up all the RF/EMI that BX does; but with isolated ground receptacles, BX is just as effective.
