I have only ever used the Garmin 178 models.
I had the black and white (gray scale) one on my last RIB and thought it was a wonderful unit. So much so I stuck with it but treated my self to the colour version on my new boat.
Its fairly near their entry level stuff but this unit with a blue chart chip in it has got everything you could ever want to know including tide prediction sunrise sunset times as well as having easy to read charts, excellent depth sounder and temperature.
Its also fully configurable - (almost too configurable) -0 in that you can choose exactly how many pieces of data you want on each screen and what data to display. It took me over an hour of playing to finally make up my mind what i wanted where.
heres a picture of the screen i mainly use - all the info you could want plus mini version of chart and depth sounder. At the touch of a single button I can go full screen chart or full screen sounder.
Like most things in life - you get what you pay for - if you want a bigger screen then pay a bit more and you get one but i personally think this unit is perfect for the medium sized RIB I have mine flush mounted in the console and it doesn't take up too much room.
And here's where i had it mounted.
cheapest place to buy Ive always found is JG TECH
who have the black and white one for around £350 depending on GPS antenna and transducer choice. They have the colour version for around £550.
Remember though if you are going to flush mount don't get the internal GPS antenna - it sticks out the top and cant be flush mounted - if you are going to mount it on top of the console with the gimbal then i would go for the internal antenna as its one less wire to have to hide. I have had one of each below is a picture of my old black and white one.