The 200 will give you a better pic of the bottom over the 561. If your a "fish ID" user (fish symbol with a depth ID), the 200's high resolution hurts that image, and you won't be able to read it...go with a 561. Then again, the 561 will show stand alone structure as a "pile of fish"
The 300 is just a 150 with color for double the price of a 150. It's only a 3.5" screen...hige resolution or not, manufacturers can only put so much on a 3.5" screen. IMO, I'd op for the 200, and deal with a "more viewable" screen in black and white.
For the nearly $500 price tag on the 550c, I wouldn't even consider looking at a Garmin product. At that price, HB and Lorance have far better products available. Garmin is a truly steller "budget fish finder". Within the $100-$200 spectrum, Garmin has some great products. Get any higher than that, the competion has them beat.