The screen size is the big issue here.
I have a Garmin 176C, this unit is also portable, but it's not easy to read the screen in bright daylight, then you will need to have the backlight running all the time, for this reason, you will need to be able to supply the unit with power from your boat. Otherwise you will run out of battery very fast.
The Garmin screen is not very easy to read, while going at 35 knots, but it can be done, just be carefull about the size.
It's like with boat's "bigger is better"