Ask Mr. Smarty Plants

The botanical description of Styrax platanifolius (Sycamoreleaf snowbell) and its four subspecies—Styrax platanifolius ssp. platanifolius (Sycamoreleaf snowbell), Styrax platanifolius ssp. stellatus (Hairy sycamore-leaf snow-bell), Styrax platanifolius ssp. texanus (Texas snow-bell) and Styrax platanifolius ssp. youngiae can be found in the online version of Flora of North America at eFloras.org. 

The major difference appears to be in the abaxial surface (i.e., underside) of the leaves of Styrax platanifolius ssp. platanifolius which are described thusly:

"...abaxial surface with white stellate-tomentose pubescence in addition to scattered, orange-brown or dark-brown, stalked stellate hairs on some leaves, surface completely covered and obscured by pubescence..."

("Stellate-tomentose" means covered with a soft, tangled mat of short star-shaped hairs.)

The adxial (upper) surface is described as being glabrous (without hairs).  Also, the leaf margins are described as "often weakly undulate."

The leaf pedicels are described as being white stellate-tomentose and the flower calyces are "thinly and evenly white stellate-tomentose, margins and teeth densely glandular..."

You can read more about the differences between Styrax platanifolius and its subspecies at the eFloras site.

If you click on Texas on the USDA Plants Database distribution map for each subspecies of Styrax platanifolius, you can see where it occurs in the state.