I have both the 520 and the 720 up and running on every flight. I am perfectly happy with both of the units. Here are the differences that I see:
1. The 720 is bigger, therefore easier to use
2. I don't think the 520 can display weather, whereas the 720 can.
3. The 720 may have a slightly more crisp display, but both displays are A+ in my book.
I really like them both, but if I had to pick just one, it would be the 720 because of the size and weather option. Also, don't hesitate to call them, they are very willing to discuss this with you (I called them). Also, I believe there is a 30 day money back, no questions asked, return policy. I also like their annual VFR/IFR subscription prices.