About $10.
j/k
A quick (and probably bad) analogy would be this:
A switch has extra "smarts" in it to open the packets of data flowing through it and send them directly where they need to go. If your iMac, Mac Pro, and Airport Extreme were all plugged into a switch, and the iMac was sending packets of Internet data to the switch, the switch would be smart enough to redirect those packets only to the port on the switch that the Airport Express was plugged into. The port on the switch that the Mac Pro was plugged into wouldn't see any of that data flowing by.
A hub, on the other hand, is much simpler. In the example above, if the iMac was sending Internet data through the hub to the Airport Express, the hub doesn't have a clue which port on the hub that the AE was plugged into, so it will send the data to ALL of the ports of the hub. The Mac Pro could actually see the iMac talking to the AE in this case.
In a small house network, I've never noticed any performance difference between a hub and a switch.
With a switch, there's a potential small performance hit as the switch has to monitor each pack of data to know where to send it.
With a hub, the more devices you plug into it (and the more data you send through it), the more likely there is for data collisions (when two devices add data to the network at the same time, which can also cause a performance hit.)
I'm sure others will chime in on this thread, but with the size of your network, either one would probably work well, and I'm not sure you'd notice a difference between either one.