Given the time window I would certainly suggest meeting at the airport, as that way you are sure not to have any issues, other than having to go back through security, which tends to be fairly painless at that time of day. However, the advantage of going into town is that you could walk down from the Cornavin train station to the Mont Blanc bridge, have a view of the lake, and then get a drink in one of the bars before going back to the station to return to the airport. Taxis are expensive and won't be quicker.