Idk Mark, I'v yet to meet a "hard to grind down" weld bead. A good, powerful 4.5" grinder with a quality wheel can make easy work of that job, and a few seconds with a flap disk can make it look like the repair never happened...
Id personally say the differences between TIG and OA are:
Tig gives more control (especially with a foot petal)
Tig puts less heat into the metal (hotter source of heat, but its much more precise, and requires no pre-heat)
Tig is faster, much faster.
But, each to his own.