At lakes like Buckeye, St Mary's, Indian, Loramie, downscan is a waste of money--period. At Alum, Delaware and similar lakes, it'll matter more, but not much unless you have sidescan as well. Without sidescan, it's just a fancier interpretation of what regulare sonar already tells you--there's something down there that's different and should be fished. Also, at high speeds, sometimes bottom lock on down and side scan units can be a problem--think 45 MPH and higher.
As to L vs H, L has better defined downscan images with dedicated crystals in most units, but God help you if have a problem as their customer service sucks and has for quite awhile. H has great customer service and is more leading edge, but has issues with their slow speed GPS directional positioning (i.e., it won't tell you what direction you're going at speeds less than about .5 MPH, a problem for slow trollers).