I would go with the made for the trolling motor transducer (XTM-9-DI-25-T).
Two reasons for this: first is that it is made to fit the trolling motor and will not hang on weeds and get damaged as easily as an adapted transom mount transducer would. Secondly is that it is built with an additional RF shield which will help if your trolling motor is radiating RF energy (trolling motor interference).

Make use of the Transducer Exchange Program when you receive your new HELIX 5 DI model unit (don’t open any bagged items until you talk to them!).