6xx vs 7xx - you get the extra 10 watts per channel and better crossover control for the speakers.
With the 70XYou get the extra 10 watts and preouts so that you can add an external amp later, and more/better Audyessy functions. I can't speak to the crossover points but the crossover points can be set individual on my 606 but it's possible there are more crossover point options on the 707 - I just don't know. There maybe other feature differences.
x06 vs x07 - you get nothing that really matters.
That depends on what matters to you.
Keep in mind that the 607 is available as low as $330 refurbished and the 707 $500 refurbished. If I had it to do over again I'd spend the extra $200 for the added Audyessy functionality and better controls.
Also both the 607 and 707 are nearing becoming last year's models so additional discounts
may (or may not) be coming.
The upcoming 608.