Deccaperms are large motors geared down to give the torque.
They are electrically very "dirty".
And very expensive.
The geared 550 is relatively cheaper and gives you the option of using 6 or 12 volts, (or any in between), on the same motor so far more flexible.
These motors pull about 1 amp in air and only 2 (aprox) in use so less power hungry.
These give similar torque.
In the event of a burn out just replace a 550 motor, yes Deccaperms do have problems too.
If I buy a boat with Deccaperms in it I usually sell them and replace with Geared 550's.
Some people swear by them, I swear at them!!!
Bob