"Io non mangio frutta, bensì verdura."
I really think of bensì as "but rather". So, "not the apple, but rather the milk" (non la mela bensi il latte). In general, bensi is used only when saying "not this, but rather that". So if you see a sentence in english that has the word "but", and could sensibly be changed to "but rather", use bensi. For the rest (and most) of uses of "but" ("I don't want to, but I will" etc.) use "ma"