Personally I find structure to be a very useful indicator in the ID of Red-necked and Little Stint. Of course, it requires experience and familiarity with the species involved.
For me, this bird has a rather short, deep and blunt-tipped bill; relatively short legs; and a 'hunched' posture with the head held almost in line with the body. All of these are features that I associate with Red-necked and not Little. Therefore I agree that this bird is a Red-necked.
Plumage in Red-necked Stint is extremely variable in my experience. Juveniles can have a relatively prominent V on the mantle I think (even if not all). Adults in spring can have a wide range of plumage pattern depending upon the stage of moult (varying in extent and intensity of brown and red tones on the upperparts and head), and this also invites confusion with Little Stint. Even after over 10 years watching waders in HK I still struggle to be sure of the ID of some individuals.
Unfortunately we don't often get the opportunity for such good views of juvenile stints in Hong Kong, because numbers in autumn are lower than spring, and the birds are less often seen at close range (because of the tide level). This is a great set of photos, and very useful for helping us understand more about this plumage.
