The advantage of having a HS GPS is that it will show what direction the boat is pointed even when moving slow or sitting still. Most people want the HS GPS at the bow since that is normally where you are fishing from if going less than .6 mph. Moving less than .6 MPH with a regular GPS will result in a "donut" zero looking symbol and loosing heading information with a GPS that doesn't have a heading sensor.
The advantage to having an external GPS on the transom for the console unit is that there is less error introduced when marking a waypoint .... but that would not necessarily need to be a HS GPS since you are typically moving faster when using the main motor.
I would put a HS GPS at the bow and a regular GPS puck at the transom.
Pre testing is important because any magnetic field will affect the accuracy of the HS. Check locations with a regular compass first and then do a temporary mount .... The HS can be greatly affected by the trolling motor when stowed. Some people don't care about that ...otheres do.
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Looking for the one that makes it all worthwhile.