The public USairways.com site shows how many seats are for sale - i.e., remaining up to the overbooking limit. Wings should show the actual number of seats available, i.e., remaining of the installed (usable) seats. So Wings (aka the employee travel site) is more accurate but since many non-revs don't list till within a few days of travel it doesn't give a good picture earlier than that while the public site is better for seats sold a week or more out. The employee site does give both the booking limit and actual seat count, so the booking limit can be useful when looking at the public site.
Jim