The problem with flap wheels is they'll ride over the bead and take metal off the panel on either side. A solid grinding disk will only remove metal where you touch it. Again, personal preference.

And I would never run straight argon on a mig, argon is for tig. Keep in mind that straight CO2 gives the best penetration BUT a lot of spatter. A mix of CO2/argon reduces penetration but virtually eliminates spatter. So it only stands to reason that straight argon will have the least penetration but no less spatter. Also, argon on a mig is only recommended to be used on sheet metal, so if you use your welder for multipurposes including thicker metals, you don't want argon. The mixes I've used are "Mig gas - Gold" and "Auto weld".