I have a Garmin 176 (mono) and a Garmin 176C (US import).
My opinion of these units is good. The 176C in certain lighting conditions (eg the sun in your face behind the unit) can be difficult to view. The 176 has better readabilty in these conditions. I prefer the 176C as the colour representaions are identical to Admiralty paper charts, therefore it is easy to distinguish depth contours without having to look at the actual figures.
The screen updates very quickly and the sattelites aquire within seconds of switching the unit on.
The supplied mountings with the Garmin 176 is poor. I can recommend buying a 'Ram' mount !!Ram mounts
This Ram mount is very robust and has served me well in very bumpy conditons on both my small Rib and Sib. The mounting can aid in positioning the unit as close to you as you possible to make up for the small screen on the unt
I have had no problems with the electrical connections on either unit, maybe a dab of vaseline on the terminals could prevent corrosion?
An advantage of this unit over larger units is that it is portable, therefore it can be used in your car, Sib and all the passage planning / waypoints etc can be entered while at home.
Finally there is no external antennae to worry about.
If you have the space you can't beat a big screen, however the smaller units do have advantages