Create a parameter that has those two choices.
Then create a calc that looks something like this:
If [Measure Param] = "Count" then [My count measure] else [My percent measure] END
Use that calc on your sheet in place of either count or percent measures.
Do yo have a simple example?~~I am a newbie on Tableau so I am a little confused about your method~~~Thank you:)
;( I did not get any luck from it. but thank you anyway.