I just bought a HumminBird 798 CiSI for my bow. I have an HB998 CiSI on the console.
The problem with having only one on the console is that you have to turn it around when you're on the bow fishing if you want to use the side/down imaging or the sonar, and then turn it back when cruising to another spot. Inconvenient, redundant, pain in the neck after a whole day on the water.
The 798 and some other models have an internal GPS antenna. If you are fishing from the bow (with the transducer attached to the trolling motor), when you pass over interesting or productive structure, you can mark it immediately. The GPS saves the created waypoint which allows you to return to that exact spot another time; no guesswork/deadreckoning.
The side imaging is used basically for "scouting" (usally from the console)to scan a large area for potentially productive areas of structure. I use the down imaging on a split screen with the sonar to pinpoint the exact nature of any structure I pass over while fishihing from the bow. Adjusted just right, the DI will give you a photo quality representation of the image on the sonar even down to individual branches of a brush pile or blow-down.
There are a number of methods to share waypoints between two units so that you can effectively use the GPS on either unit to return to productive spots that you have marked at different times on one unit or the other.
If you do purchase a unit with an external antenna, the accepted train of thought is to mount the antenna as close to the loaction of the transducer as possible. This provides a more accurate marking of waypoints. If you are on the bow, and your transducer is on the transom you are gonna have (at least)16ft of offset by default. Keep in mind, that may require drilling holes in the deck of your boat.
To bring all of this together; if you are serious about catching more fish, spend the money. If you want GPS, but don't want to shell out $700+, there are units that are less expensive that have the GPS/Sonar Combos but no DI/SI.
Hope this helps.