Not trying to be too much of a smarty, but my answer is the one that fits in your price requirements and the one that you will practice with to learn how to use.
I could easily say, "Oh, Oh, Oh, buy the one that I bought." Might sound like a funny comment, but if your best friend owns X unit, it's not a bad idea to buy a unit that has a built in resource of information available.
A comment about size. Don't think that you can only buy a 4 or 5 inch unit because it is a kayak. I'm running a 7" unit and a 9 or 10 wouldn't be too big. It's nice to have the extra real estate when using the split screen technology.
I think that side imaging is a grand thing. Pick what you can get the most bang for your buck TODAY. Sometimes the Birds are a better price than the Lowrance units and vice-versa.
Many go fishing all of their lives without knowing that it is not the fish that they are after. Henry David Thoreau