This is the same decision that is plaguing me! OAG or piggy back scope?
With an OAG, (please correct me if im wrong) you can only use a small percentage of the field of view to find a star to guide with?
Where as a piggyback scope you will have a greater number of usable stars to guide with?
Here are 3 videos i watched, that helped explain a lot!
http://www.youtube.com/watch?v=YK0eLznxGh8 <-Piggy Back
http://www.youtube.com/watch?v=VWXhVyKQk88 <-OAG
http://www.youtube.com/watch?v=8TClzFbxwhM <- Very informative !!