The default will usually be the fastest your machine has which should be the N. My router has separate radio's with separate ssid's so I can direct the device to to the N.
I understand that N is faster and that the iPad will negotate to the fastest connection. I was trying out the new Cisco E2000. But I wanted to see what the iPad was reporting my speed and connection type was.
Usually your broadband connection is the bottleneck rather then G or N anyway. Unless your sharing over your local area network (movies, large files) the local speed of the wireless will far exceed your broadband internet connection.