The Wahoo mght have the most readable screen.

The 520 might generally be a bit harder to read than the 510/500.

The Wahoo and the 520 have similar features. The Wahoo displays much better maps and the 520 has (I believe) better exercise/performance related stuff.

The navigation stuff is similar (but the Wahoo might look better).

The new 520 adds the more sophisticated navigation that the 820 and others (units with maps) provide.