Advice between Humminbird 160 vs 170

If all your experience is with high-priced fish finders, you can probably skip this thread :o

Here's the quandary: I can either get a Humminbird PiranhaMax 160 Portable (includes carrying case, battery, suction-cup transducer, etc) for $120, OR get the 170 version but spend $180 by the time I've outfitted it as portable. BTW, portable is a necessity for me; I don't have a boat, but have use of one that is for the public.

The main difference between the two is that the 170 has eight levels of grayscale vs. the 160's 4 levels of gray. (the lake is only 15 ft deep so either unit has enough power).

Is the higher level of grayscale worth $60...when you consider that $60 buys a nice ultralight rod.

Thanks for the advice.