The round would appear to have more area than an octagon at first, but since PCB layouts are basically square and governed by clearances a square pad would maximize area for a given pad-to-pad clearance. The etching process as well as noise concerns traditionally frowns on 90 deg features and it's less efficient to route wires around sq features. So the octagon is usually used instead of square, and octagon wins over the round every time.
Now octagon vs long is just about what kind of clearance is available. A TO220 pkg for example often requires longs because the package gets mechanical stability from the pads. DIP pkgs, nice but not necessary. Sometimes you have to place stuff so close to a DIP pkg that the longs won't work. Not a crime to use octs.