I voted fingers because they are so variable, can be hot/cold firm/soft rough/ gentle and so on, but instruments are a lot more static and very similar with different users. but fingers are always different for each user.
Its down to choice as a lee i prefer a ler who uses fingers.
Having said that some instruments have been good sometimes its down to mood, who the ler is, how well we know each other and get along together, and where the instruments are being used, and all the other hundreds of things that make tickling a pleasure or disaster.