I have two wifi hotspots set up. Comcast X1 Router (Wireless Gateway 1) and an Airport Extreme. Both networks are set up with the same SSID, passphrase, and WPA2 Personal.
Unfortunately, the wifi switch seems to work only in one direction. Initially I will be connected to the Airport extreme, as I get in range of the Comcast Router my Macbook (and iPhone) automatically switch to the stronger signal. When I move back to the airport extreme I stay connected to the Comcast box with inferior signal strength. I verified the signal strength with Wifi Explorer.
Turning the wifi on and off when close to the Airport Extreme will reconnect me to the Airport Extreme.