It would strongly recommend doing a bridge, there's little reason not to. If you wanted to do it with routing tables, you'd have to set up a different block of IPs for the client. For instance, on one of the gateways, you'd have routes like this:
With eth0 being 192.168.1.1 and wlan0 being 192.168.2.1. You would make the client laptop be 192.168.2.2, and it would be routed through eth0. If this doesn't make sense, I suggest using bridging. It's easy, and I can show you how if you want.