The transom is definitely easier. replacing the old transducer can be easy or difficult, as it depends if the size if the new is the same as the old.
As to the position of the transducer in a transom mounted config depends on the bottom surface of the boat. You want the transducer in a location away from tubulance caused by the prop and mounted at a spot where any chines or strakes on the hull surface will not disturb the water flowing over the transducer. There are recommendations and guideline in the installation instructions of the transducer.
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Jvalich
http://www.badcock.com'04 FW 288 Vista "Mental Floss"