Which size sounder/gps

Nice selection on the FF.  I have the same thing but without the GPS portion.  When you put the units side by side (color and grayscale) you can really see a difference, but maybe that's just my eyes.

Also, the additional battery drain isn't really an issue.  A 5 or 7 amp hour battery will last all day. 

Yeh, What kallitype said... you dont need to change out the transom 'ducer for something else... not unless you want to.  nice to have but like yarjammer said the otehr day, it's a waste of your money.