My decision would change depending upon the situation.
Provided it was my lover, I would definitely choose A. Feathers don't tickle me too badly, but I am extremely aroused by the idea of being tickled naked with feathers. Then again, some very....secret....parts of me are highly feather-sensitive. Plus, being naked, bound, and tickled in any fashion by my lover is just an attractive concept. =P
If I am to be tickled by a friend, or more importantly, someone I don't know, I would definitely go with B. I really like to be tickled gargalesis style, but I would much rather be clothed if it was someone I didn't know that well, or not at all.