I went with what I thought was modest and not so "out there"... a C cup on my 42" chest. I do like them but after a while I decided they didn't match my upper body and to 'pass at a glance' I need more.
I recently got a set of D cup forms and what a differance. I do enjoy them more, that extra weight and the sensation of having more 'out there'.
And as for appearances, I do feel they do really make a more appropriate silhouette for my stature.