Need some help with a sounder purchase, I am looking at the Garmin echomap 70dv at the moment as I can get the unit for a reasonable price and later add the side vu/down vu module. Just have a few question in regards to it and thought that there maybe some experts/owners out there. What is the difference between it and the echomap 70s? How does it compare to lowrance and Hummingbird? What are the preloaded maps like as I haven't seen Bluechart before? Any help would be greatly appreciated as it is giving me a headache.