I'm looking into putting a unit on my boat, I've never owned one and have fished without one since I got my boat 3 years ago. I do alright on catching crappie but do move around a lot and use navionics to get a idea of the depth I'm fishing. But don't really know which unit would be the best for just crappie fishing. Which is better di or si for locating structure and bait? Really don't want to spend over $500 on a unit. Would like some of you guys opinions on this.