One aspect of Frame Saver is it's ability to coat the interior without it's liquid running over all the surface. It is better to insure the liquid does run over all the interior but reassuring that the aerosol will reach spots that one's various positioning of the frame (to get the liquid to run over all) might miss. I don't know if T-9 does the same.
But so much of this is like the "which chain lube is best" discussions that return so frequently here. I tell my customers that it's like bathing one's body. That you are concerned and use some sort of cleansing "soap" is the real need, not which "soap" you choose.
So use what you find pleasing and agreeable to your sensibilities. Don't worry the fine points, just using some sort of rust inhibitor is the start. And repeat every so often, more frequently if you ride in rust belt winters, coastal salt air or through floods
As to how much to use- enough to have it drip out all over the place there's a vent or opening. With Frame Saver that's 1/4 to 1/3 of a can the way I do it. With T-9 I can't speak to. Andy