Fish Finder Functions

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!

To expand my question further, is Side Imaging worthless with a trolling motor mounted transducer?

No it is not worthless! !! I have two 998c units with si on both with each having their own si transducer. I love si on bow on trolling mtr. If you zero out on waypoint and it's not there, you can turn on si and find it by turning trolling mtr. It's almost like having 360

I did not consider that. Do you think there will be enough detail on a 5" screen to be useful?

Yes. I used to have a 798c si and it worked great for a bow mount unit

But if you are doing a lot of rigging, then 2D and DI might be the best for you at the front of the boat. I think a lot of riggers rely on the 2D to warn them of brush coming so they can lift rods before getting all tangled. The cone from 2D can let you see what is ahead of the boat by a few feet.

The si units I'm referring to, (Humminbird), have 2d & di built into the transducer

And not amp!

For the open water rigging I do most of the time I run a monochrome 4 1/2 in and find it sufficient. Mostly I'm looking for fish depth or shad schools so I know where to set my lines. If I were to upgrade it would be a 7 in 2d/DI with chart. I'm kinda unique since I fish from a stick boat and have my 998 mounted right by my right knee with ducer on transom.