Wonderfalls was actually supposed to debut the same fall (2003) as Joan of Arcadia, but was postponed for a year.

Joan is spoken to by God appearing as regular people. Jaye on Wonderfalls is spoken to by inanimate objects in the form of animals.

Joan is in high school, Jaye graduated recently from college. Both Joan's and Jaye's  families are involved in the series and both women become romantically involved (with Adam and Eric, respectively).

Both Joan and Jaye think they are going crazy at the beginning and each end up seeing therapists.  Wonderfalls was canceled part way through its first season (4 episodes aired, another 9 are on dvd), though future plans were to have Jaye institutionalized -- which is reminiscent of Joan's Crazy Camp (which unfortunately, we never were shown). 

Both Joan and Jaye are given suggestions of things to do, which are open to interpretation. When followed, these suggestions usually end up helping people in some way.

Although Wonderfalls only had 13 episodes, it finished its story arc (at least for that season). Joan of Arcadia ended both seasons 1 & 2 on a cliffhanger note.

Steve