ok so real quick lets look at what disc brakes change , and allow possible .
first thing that they change is stopping power , a quality set will give you smooth powerful stops with very little tire skid , ( see cgn video ) but will add weight which means less efficiency and a potentially harder ride !
next point for discs are they take the wear and tear off the wheel , a bit more , the wheel might last longer , becuase the disc rotor wears out over the rim brake track .
also no heat goes into your bead and tire area , making long descents worry free , and thats basically what they are best as is taking the wear point from the wheel and putting on the rotor , the single most import reason to use discs after safer stops is less wear on your wheel , allowing for longer wheel life .
either way will work in theory , rim brakes are lighter cheaper , some what easier to use , when considering the hydrolic lines on some discs , and also dont need special shifters , so its more like Di2 or etap , you dont need it but its good to have , in certain situations ..
all i know if my friends 10,000 madone disc is heavier than my madone 7 rim , and is size small , so im happy with rim for now !!!!