WEP came out with the wireless standard B. Very few cards B support WPA.
WPA came out with the G standard.
WEP can be broken into in 5 minutes (search google videos and they have a film on it being done. WEP also has a larger overhead and than WPA which can cause some decrease in speeds.
If you are using G you should use WPA, if you are looking into B WEP is fine. (its better than nothing) Most of the people see a network encrypted (which im sure more than one network will show up on a scan) the user will ususally not waste time trying to break into a wireless network when there is an open one aviable.