MEGA DI units come stock with the XM 9 MDI T transom type Dual Spectrum xducer ...
The TM version is XTM 9 MDI T ...(which is the same MEGA DI xducer in a TM puck housing) ...
https://www.humminbird.com/Products/XTM-9-MDI-T/
XTM 9 MDI T will have extra shielding for possible RFI and most likely will have a shorter cable (15ft) ...
Rickie