$59.99 now, but still a good deal...
Which Harmony to buy?
Comparisons, pictures etc
http://www.remotecentral.com/cgi-bin/mboard/rc-harmony/thread.cgi?4251
Essentially - they're ALL about identically capable. If you own one, don't 'upgrade' unless you're a me-too color screen desiring wannabe 
If this will be your first, buy whichever
a. is on special
b. has a button layout you like
FYI, the 659 has the transport buttons (play, stop, FF, Rew) on the very bottom of the remote. The 676 has moved those transport buttons to the middle (much better placement).
The 676 also has hard A & B buttons for DVR usage