this so called callier effect and this change in curve is another approach. as donald says - u can use it for creativity, or u can re-adjust your exposure/developing.
as a philosopher i will say - theories are great thing and great fun to deal with, but be practical (yes, based on your theoretic knowledge which primerly comes from "practice" and "logic")
in practice - both light sources have their own characteristic which can be used by the printer. it is like a differance between the fender stratocaster and the gibson lespoul guitars... which one is best???
come on - this question is not serious - i love both handricks and jimmy-page sound and gilmour sound and many others.
wether u r roll or sher film user - these heads will give different character. if u do prefer one of them - based upon your expiriance (and not theories whithout a real visual sense of the light sourse outcomes on the print) - if u prefer one of them, it ok too.
it is like to ask which leica lense is more normal 35 or 50. i dont know actually. the 35mm is more rewarding to the space perception and delivers more dynamic sense. the 50mm is more normal in terms of human eye magnification ratios of objects in different distances, and the perception of space is more concentrated, giving a more solid sense. u know how to use them and u use them based on your feelings, senses, intuitions and needs.
in creative photography (and anything which is concendered to be creative) - differances (in nature and essesne of things) are not a matter of advantages or disadvantages - differances are options, and options extened the creative abilities if u know how to use them and how to explore them.
there is some "field" where the print on the condenser and the print on the diffuse will be hardly difined, but there are huge fields where each one of them demostrates its own character. if one want to choose only one head, he/she should consider which direction is prefered.
personally, i love both of them. and if i have to gave up on one of them - it will be very difficult situation, but i would rather stay with the condenser. but this based not on "dry theories" but on my vissual sense and practical use - there is a unique character of condeser that i wouldnt gave up.