I'm new to spider rigging/trolling. I'm ready to upgrade electronics but not sure what functions are beneficial/necessary for this type of fishing. I have a 99 Tracker 175 with stock fish finder at the console. I want to add a unit on the front deck. What do you have and why? What functions are most helpful at locating fish; SI, DI, etc... I would like to stay in the $600 ball park. I can pick up a 598ci HD SI for about $500 but is SI isn't helpful then I could pick up a 7" unit with DI only for not much more. Thank you!