Yes, Ar is heavier, but not that much heavier. After all, Ar is a stable componant of air (0.93%), just like N2 (78.09%). If you charge the headspace of a paint can with Ar (which is why I keep Ar in my shop), it settles out of the air and down to the surface of the paint to prevent the paint from skinning over --but that only works in a sealed can with a hugely Ar-rich atmosphere; in an open container, the Ar just mixes with the air. This is why I shake my head at the idea of CTB with an open or nearly-open container of Ar: all you'd get is a free-mixing Ar-rich aerobic atmosphere with plenty of O2 contamination.
Anyway, my sense is that there'd be no effective difference in method or equipment. The elastic at the bottom/neck of the bag is pretty snug, so I'd guess it'll still take the pressure of the inflowing gas to push the Ar out --it isn't as though there's a lot of leakage otherwise.
One advantage is that all the cheap regulators are calibrated for CO2 and Ar, so there'd be no wondering about the accuracy of the regulator as there is when using that regulator with N2.