These gentlemen wear the olive drab wool M1912 service uniform. The NCO on the left wears his with the pullover shirt, also of wool, and the lemon squeezer campaign hat. The latter has cords in the branch insignia, in this case cavalry yellow. Both men wear the 1917 canvas gaiters with leather reinforcements issued to the cavalry. The NCO has a badge beneath his chevrons which, unfortunately, I cannot make out.

For further clothing reference, here's an extant uniform in the Smithsonian:

http://americanhistory.si.edu/collections/search/object/nmah_449369

Vols. 3 and 4 of Randy Steffen's The Horse Soldier covers the period in question, and can provide more in depth information on badges, uniform clothing, &c. Sadly, these do not contain photographs, only line drawings by the author.