The most obvious difference when comparing the logitech G502 vs G602 is that the G602 is wireless. Now for a lot of people, wireless gaming mice are a big no no. But for me personally (a casual gamer), I don’t see this as a problem and I love not having wires interfering with the clean look of my setup.
A second difference is the DPI settings. The logitech G502 has a range between 200-12,000 DPI whereas the logitech G602 only has a range of 250 to 2500. Now, this isn’t a problem for me personally as I can’t imagine a situation where I would really need 12,000 but maybe you have a purpose for it.