While I have not read the particular books that you are speaking about, I have read several other accounts. My impression is that the authors of such books are the ones most responcible for the idea that one certain group of soldiers were "tougher" than another group.
Certainly all of those soldiers were far "tougher" than most of us are today, and Arkansas and Texas were tough places to live in the 1860's. Life in those times were tough everywhere as a matter of fact. This is not to detract at all from the heroic deeds of those men. But most often those deeds were inspired by boldness of those soldiers, and the elan of the unit's esprit-de-corps. Those things most often comes from the leaders and leadership of those who command such men.
As such it is my impression that the difference between Arkansas and Missouri Cavalry and Texas Cavalry can most directly be traced not to the troops themselves but to the commanders who got the most out of his men.