I know it's the length of the boom when you see for example a 315 cat with a 10' 2" and then the same kind of machine with a 8' 6" stick. I just always wondered which one would be the best. Is this an option for different kinds of jobs and uses or what? I would want one mainly for grubbing trees and cleaning ponds.
in general you would use the shortest boom & stick available coupled with the largest bucket that is correct for the material density you are moving to move large quantities of dirt fast. If you want reach you need a longer boom/stick combination and a smaller bucket (to avoid making the excavator unstable). No "one size fits all" I'm afraid. For most models short/medium/long booms are available coupled with a variety of stick lengths. To figure out the correct size bucket for any boom/stick combination you have to refer to the specification sheet for the machine coupled with the material density. An excavator equipped with the shortest boom & stick combination is often known as a Mass Excavator (ME) configuration.
The company I did work for had a 460 Volvo with a short stick/big bucket. When loading trucks, you had to keep your trucks close to you because of short stick but it sure made up for it with power/quickness. Pound for pound it was one tuff machine