I did quite a bit of research almost two years ago when I was buying a 898 for the dash and a 788 for the bow. I hear mixed opinions on the SI up front.  Some say they really like it.  Thing is you pretty much have to keep moving and in a straight direction for a good picture.  If you swing one way or the other one side will distort.  On SI they say to try and match your chart speed with the speed of your movement for the best results.  Ex moving 4 mph with the big engine then set chart speed at 4.  Of course it would have to be slower for the TM.

One thing to remember.... you want the antanea or gps as close as possible to the transducer .  So for my 898 at the dash I mounted the Transducer on my jackplate and the GPS puck is mounted on the back deck near the engine on the same side as the ducer. The farther apart they are the more error you have. For up front no matter what unit you go with, just get the internal gps and put the ducer on the TM.

Another suggestion....  checkout Transducer Shield and Saver.  They are a little pricey but I have no regrets on their products.  They are extremly sturdy and the ducers don't move at all  !!!!  I just had to let them know the brand of my jackplate etc and they set me right up with what I needed.  Shoot it even came with a template for laying out screw holes and such. They are well worth the money considering that most transducers for SI run from $150 to $200  !!!!!   And I have broke off a couple on my TM before !!
 
If you want a source for more information, message me and I can help ya out.