Find what works for you. Run it, look at the output, see which one you prefer and go with it. If it were me I would go with either the TTY or FROM column in w. Either way it will let you know who's sitting at the computer. The user who has a tty# instead of a pts# under the TTY column is the user sitting at the computer. Or the user who has a local display (:0, :0.0, :0.1, etc) instead of a pts# or remote IP/hostname under the FROM column is the user sitting at the computer.
See which one you prefer on your network with your machines and your users. The advantages and drawbacks to each method have been provided.