Kinda makes a difference what you want to do, I think.
It's the fastest refresh rate option but that's not gonna make a lot of difference up front cause the troller ain't gonna outrun any of the rates so the internal antenna is good there. For marking a spot as accurately as possible, your bow unit with internal antenna is gonna be darn close over top of the front transducer and what I mark from up front usually relates to something I'm marking on the bottom so again, I don't see needing an external for that.
With the console unit, the refresh rate might make a difference for the go-fast rigs. An external can be placed at least 5 feet closer the the rear transducer that the console but my understanding is the standard for that placement is up behind the driver seat on the rear deck because of distance recommended from magnetic mass, both metallic and electrical (engine and charging system).
It will have to be calibrated once installed.
I don't see advantage for 2 of 'em but that's according to my use. Your mileage may vary. Refresh is main plus but I'm not aware of accuracy advantage.