I'll throw in my humble 2 cents here and place my vote for the Humminbird units. I'm not anywhere close to the expert level of knowledge that Wayne provides here, but I have been studying sonar units for going of 2 years, and have been an owner of an 1197c si for a few months.
I could not be happier with Humminbird for several reasons. Not the least of which is their customer service. In a word... Excellent.
The unit itself is very easy to use. There are some very good forums online to answer even the most technical questions.
You can purchase a used unit, and upgrade the software to get the latest version...for free.
Also, I (or Wayne) would need to see a pic of your transom to be sure, but I believe you can mount a transom Side Imaging transducer in a way that nearly eliminates the chance of ripping it off on a tree or stump. You might reconsider the Side Imaging option. It's definitely worth the money. Lightly used 798's have been selling online for around $700.