You'll want the 56 transducer. I believe the lowest price right now is West Marine, Garmin is sold out and Bass Pro is the GT54 unit. Garmin recently upgraded to the UHD2, better graphics supposedly and a few other features. Basically it's the same unit only repackaged, I believe.
Nice feature about the Echomaps is they will drive the autopilot and don't require a head unit provided you go Garmin AP.
My son just upgraded his Garmin with the 94 UHD / GT56 and he is super happy with it.
Tight lines...