If you can get to a place where you can play with them, like Cabelas, it would be a big plus.
I just got done installing a Humminbird Helix 7 CHIRP sonar GPS on my 1648 Tracker, it had an Eagle Fish Mark 320 on it when I bought it and I didn't like that unit at all.
I fish a river with that boat, 3-12' depth mainly, with a few 30-50' holes, I'm more interested in the depth than anything else and finding drop-offs etc., so in shallows it's a depth finder and in the deeper holes it's a fish finder.
A small unit would probably do what I/you need, but I like the bigger displays. I didn't go with SI for the river, maybe would have considered it if used more on a lake.
Cabela's and other places sell a clamp on portable transducer bracket that seems pretty solid, might look into one of them for the stern, which is where I would mount a SI transducer if I were to ever get a FF with that capability.
Just some thoughts!
BTW: I didn't really need the GPS, but the river has a 15mph speed limit on it during the summer, so it was either get one with the GPS for speed or buy an optional kit for it to show speed.