In addition to Max's reply:
Most Excel Worksheet Functions are not case sensitive.
Most VBA Function are case sensitive.
You posted to worksheet functions but perhaps your
question was really a programming related question (VBA).
One way to do a comparison in VBA would be to make
both either uppercase or lowercase.
IF UCASE(var1) = UCASE(var2) THEN
Some more information on VBA text comparisons
Text Comparison, Case Sensitivity (#compare)
http://www.mvps.org/dmcritchie/excel/proper.htm#compare
The following VBA comparison is faster than using UCASE or LCASE
MsgBox InStr(1, Range("A1").Value, "banana", vbTextCompare) > 0 'insensitive
also of interest for Worksheet Functions
http://www.mvps.org/dmcritchie/excel/strings.htm
You might also gain more insight by using Answer Wizard
tab in HELP both for Excel and later for VBA searching for
case sensitive