A flex plate for an automatic transmission is a thin piece of steel plate with a starter ring gear welded to it. The torque converter is bolted to it and provides the inertia to keep the crankshaft spinning smoothly. Standard shift flywheels are about an inch thick, and weigh in at 30-40 pounds plus the rotating weight of the clutch and pressure plate, so whomever told you they're the same has his "head up and locked" as we used to say in the USAF! On an internally-balanced engine like an inline six, there won't be any difference in the balance of a flex plate or a flywheel.
Jerry
