iPhoto vs Picasa

The short answer is: Picasa.  Here's why:

1. Face Recognition.  iPhoto pretty much only recognizes that there is a face in the photo.  Picasa does an impressive job of figuring out who's face is in the photo.  This makes categorizing photos much faster.
2. Files.  iPhoto requires you to store all your photos in an iphoto folder where it hides all the file names for no reason that I can figure out.  Apparently, if you can get used to it, it's not so bad. 
3. File locations.  iPhoto offers no simple way for a user to indicate where the big blop of photos that iPhoto will make is located.  I like to have all my media on an external drive.  That's not an option with iPhoto.  Note: there is a way to configure it from the command line, but why bother given items 1 and 2 in this list.

Overall, iPhoto is kinda lame when compared to Picasa.