I do not know very much about the Black-capped conure or the parrotlet, however, before getting a conure you should be aware that they are usually very loud. My fiance's parents own a Sun Conure, and he is much much louder then a Cockatiel could
dream of being :-). Saying that they are quieter then 'other' Conures does not mean very much to me at all.
Just a thought.. have you considered a Senegal Parrot? Perhaps I am biased, because my Hermes is the love of my life, but they are some of the quietest birds I have ever experienced. They also tend to be very sweet. Many of my friends own Senegals (b/c they loved mine so much) and they all seem to be incredibly sweet and tame. I have also not noticed my Senegal bonding with just one person and disliking others in the family.
Perhaps you should just look into the Senegal Parrot. If you want a very quiet smaller bird that will stay sweet you should definitely consider it. Here is one expert's opinion:
"Hand fed Senegals make extraordinary pets, and are known for being comical and entertaining. They are colorful, relatively small, and have the ability to talk and mimic, although they tend to be considerably more quiet than many other parrot species."
This is from:
Senegal Parrots You should check it out!
Sorry... I didn't mean to steal your thread or anything, a parrotlet or a black capped conure may be a better choice for you. :-)