it's been a while, but IIRC the total time is fairly similar but the potash road is more time on rougher road and less time on pavement(?). i usually drive in on potash and out on that steep switchback road, i think shaefer(?).
I went down the Shafer trail, took about 3 hours from Moab to get to Monster tower. I would take this route mainly because I hear both ways are the same distance time wise, plus the view of the white rim at the top of the switch backs is pretty killer.
It's undoubtedly faster to go down the switchbacks from I.Sky. I've done it both ways and it seemed like it added an hour of driving to come in from the Potash road, it also had the roughest section of road of the whole drive as you near the junction with the Shafer Trail. Either way you go about it, it's a long, slow drive. Bring beer.