You are correct, it is The Measure of a Man. Data is being asked to submit to a dangerous procedure for the benefit of Starfleet, and questions why biological beings are not subjected to this.
From http://www.st-minutiae.com/academy/literature329/135.txt. Note: the final conversation (as shown in @PeterSabers answer) differs slightly from the original script.
PICARD
It's precisely because you are
a Starfleet officer that they can.
We take an oath to serve. In this
case this is the form your service
is taking.
DATA
Sir, Lieutenant La Forge's eyes
are far superior to human
biological eyes, true?
PICARD
Yes.
DATA
Then why are not all human
officers required to have their
eyes replaced with cybernetic
implants?
Picard is utterly at a loss for words. We can see the
confusion on his face as he struggles for an answer to
this unanswerable remark. Data rises with great
dignity.
DATA
(continuing)
I see. It is precisely because
I am not human.