Western or Semipalmated Sandpiper.

I am not 100% certain but I would appreciate any comments or help. I think that this is a Western Sandpiper but it could be a Semipalmated Sandpiper as they are very similar. (But surely not identical). Of the 5 North American "peeps", only the Least Sandpiper has yellow legs and the other 4 have black. The western and the Semi-palmated are the smaller of these 4 and are difficult to seperate. My gut feeling is that this is a Western as I think they are more brownish than grey and I am told that the markings on the western continue down the breast but please help if you can.

A bird that is easier to recognise is the Willet but even this species can be sperated in to two sub-species. I am quite confident that this is a bird of the western sub-species because of its slender bill.