The difference is the external and internal reciever and the dual frequency transducer. I had the the grey scale Lowrance M56 GPS/Depth Finder with the internal antenna and did not have any issues.
I just picked the up the Lowrance 334C-igps for less than $400.00 today at Fisherman's marine in Portland, OR. It has a color 5 inch LCD and has both the internal GPS antenna and depth finder.
It is a discountinued model, but for that price with a color 5 inch screen. I couldn't walk away. The thing is with internal antennas you need to make sure that the unit is not in an enclosed area. That is where you run into issue finding the sat.
You can also download the simulator software that will run on a PC. So you get can the look and feel of the device before you even buy it.
This is my old boat and Lowrance. The link above is the one I bought today.