Use the one that you have setup in your router's configuration.
WEP is fine, keeps casual hackers from using your network for BT.. or other devious purposes.
WPA is more secure, but the setup is slightly more complex.
Personally, I just use 64bit WEP because there's really no reason to use 128bit, and WPA is not supported by all devices. Plus you can use a telephone number as the hex WEP key too..
