The main is for power, the jib is for speed and pointing. Useful rule of thumb for all boats.
What do you mean by power and speed? What is the difference? I have always equated Power to some extent with speed.
My boat is set up with mainsail and 135 genoa. When sailing at any points excpet a run/broad run, the jib will power the boat faster alone than the main alone. This is especailly true on a close reach or close hauled. In steady 15kt winds, I may be able to do 3.5 kts close hauled main alone and 7 genny and main. With genny alone I am in the 4.5 to 5 range, but the boat isn't as well balanced and I can't point as high. On a run or very broad run, the jib is essential useless as it is blanketed by the main when both are up. It's easoer to roll ing th genny, than it is to drop the main, so that is what I do. I may lose a little in speed, but since I am not in a race, I don't care..
I always thought (was taught) one can't point as high with a jib because the main has a third fixed point to the back of the boom (clew/outhaul) that a jib/genny does not. The fixed point means you can shape the sail closer to the wind and therefore point higher.