It is true that a Down Image system will not give you a picture quality image when sitting still ... you do have to be moving. At that price range, I agree that HB vs Lowrance is a coin flip decision. If you use the factory settings (default) and don't want to go through a learning curve on how to operate a depthfinder system ... I suggest getting the biggest screen b/w or color 2d depthfinder that you can find in that price range.
While I agree that the image won't be all that good when sitting still, using a DI unit ... the stained water part of the salespersons statement is bogus IMHO. A Down Image unit will show picture quality images just fine in stained water, but you do have to be moving ... otherwise you're going to get a repetitious blur across your screen. Suggested speeds for high quality images on a DI unit, are 1-8mph. But my Elite 5DSI has shown me very good images at speeds less than 1mph ... on Herrington, Green River Lake, & Taylorsville, and I don't consider them "clear water" lakes.
... pappy