What is the difference between the 500 and the 700?
the BIG difference is that the 500 is not PC programmable, it also has less capabilities. But you can read all the details by clicking on review and then complete list and compare the 500 to the 700. Also they are both being replaced by the 650 and 850.
How durable are the Harmony's and how is their battery life?
I have a 768 that I bought when it came out (I think that was three years ago) and never had a problem, battery is not bad, but not great either (definetly in months instead of years)