Let me start by saying that Down Imaging must be used in conjunction with traditional sonar! Down Imaging only will require a steep learning curve to interpret what your seeing and can be very frustrating.
I for one am a fan of down imaging/side imaging/360 etc. The more data points you have the better you'll be able to interpret what your looking at. Since you just got a PA12, she deserves a nice sonar setup. I used to have the 598 HD SI Combo on my PA12, but upgraded to the 798 to get side imaging. Down imaging was cool, but add side imaging and you've got a better picture of what's around you. Note, that DI/SI is basically limited to about 100' on the 598 and 150' on the 798. If you're spending your time fishing deeper waters, go with Lowrance to get the 50/200 transducer. Another plus for Humminbird is the software to playback your sonar data on a PC at home, including DI and SI footage. If you're a sonar geek you'll like these options, that's why I went with Humminbird.
My 798 has the same current draw as my old 598, which would lead me to believe there is no added penalty for Side Imaging on the compact combined transducer. My 9AH battery lasts all day with screen brightness at 10. However, there is no day two without a recharge.
After your purchase, search Doug Vahrenberg on Youtube and prepare to learn how to use your sonar.