Mazzuca,
Thank you for your inquiry.
Neither function is "better" as they both can be useful at different times and under different scenario's..
The manual range / shift functionality would allow you to zero in on one particular area of the water column (possibly where the target fish species are suspended off the bottom at a particular depth).
In auto range the sounder attempts to always depict the sea floor as the bottom depth changes. The bottom zoom is useful for closer inspection of the bottom & fish species that tend to stay close to the seafloor. For bottom fish you might find white line function helpful as well. This depicts the strongest signal (usually the bottom = burnt red color) as white so as to help separate it from other targets close to the bottom.