Lowrance does do side imaging. I'm not sure where you heard that or if I'm understanding you correctly.
They're both great systems. I've heard lowrance has a slightly crisper image. I prefer humminbird though due to the user friendliness, much better mapping, and the performance is still great.
An 899 would be a great unit but I suggest finding a used hd 998 instead. Better transducer and has a remote/separate gps puck rather than it being built in. If this is going by the trolling motor that's a moot point.
I run a 998 non hd at the wheel and a 798 at the trolling motor. Screen size is paramount so get the biggest you can afford. Some may argue that but there's no logic to back up why a bigger screen is not better. I'd run an 1198 if I could justify the expense. I run lakemaster cards in my units and for our lakes they have the best detail and coverage.
Any specific questions just ask!