Pectoral Sandpiper (PSP) have an unmarked belly which is sharply demarcated from a streaked breast. Sharp-tailed Sandpiper (STSP) have some dark markings along the flanks. The 10/10/13 bird shows clean flanks, and a sharply demarcated division between a white belly and streaked breast. It is impossible in this picture to see the full extent of the breast however.
Without a clear view of the demarcation between breast and belly other features must be relied upon. A close examination of the head I think provides enough evidence for a confident identification.
PSP show a less distinct eye ring than STSP. They also have no rufous in the crown, which STSP usually does. STSP also have a darker and better defined band on the ear coverts. The base of the bill of PSP is yellowish brown or greenish brown, while STSP has a pinkish base to the bill. STSP has a whiter and more distinct supercilium. STSP also averages shorter bill and shorter neck than PSP, and with a slightly flatter crown.
edge of the mantle and upper scapulars are typical of juvenile PSP, though I have seen photographs of STSP with similar, though less distinct patterning.