Frank that is one of the most opinion based subjects going. The quality of both brands is limited only to the price you are willing to pa. The price should be dictated by what you wish to accomplish, find fish vertically, horizontally, or some variation of the two. The depth finder should be easy enough to use that within a few trips the user can be proficient enough to be successful. Now-a-days, the quality of Hummingbird and Lowrance are both very good, and both do an excellent job in all the categories mentioned above. it is a matter of choice.
That said, I use Lowrance and I am also cheap! lol. I have the 520 on my console (rear) and the 480 on my trolling motor. I am old school and fish vertically when I use my depth finder for fishing. Other times,like springtime, I use my depth finder for depth proximity and water temperature (is the water warming up enough to reposition bass like pre-spawn) and finding ditches or highways to spawning flats).
That's just me though. I hope someof the guys with side imaging units to explain thier thoughts.