How to have phone prioritize encrypted Wi-Fi hotspots?
You could change the guest network to be low priority by editing the supplicant .conf file (probably requires root, or adb use).  You can find out what file this is by looking at /etc/wifi/wifi.conf; the file you need is set as the SUPP_CONFIG_FILE property's value. Normally it's /data/wifi/bcm_supp.conf or /data/misc/wifi/wpa_supplicant.conf.  You'll see entries like the following in that file:

network={
    ssid="network name"
    someotherproperty="some other value"
    priority=0
}


You want to have priority=0 for the networks you least want to connect to, and a higher priority for networks you do want to connect to.  If you just have "don't want" and "want", use 0 and 1.  If you have "don't want", "not preferred", "preferred", and "super-preferred", use values 0, 1, 2, and 3 respectively.  You get the idea.