Попробуйте этот UDF
Sub Test_Difference_UDF()
MsgBox Difference("C1, C2, C3", "C11, C2, C3, C4")
End Sub
Function Difference(txt1 As String, txt2 As String) As String
Dim x As Variant
Dim y As Variant
Dim f As Boolean
Dim s As String
Dim i As Long
Dim j As Long
If InStr(txt1, ", ") > 0 And InStr(txt2, ", ") > 0 Then
x = Split(txt1, ", ")
y = Split(txt2, ", ")
For i = LBound(x) To UBound(x)
f = False
For j = LBound(y) To UBound(y)
If x(i) = y(j) Then f = True
Next j
If Not f Then s = s & IIf(s = "", "", ", ") & x(i)
Next i
For j = LBound(y) To UBound(y)
f = False
For i = LBound(x) To UBound(x)
If y(j) = x(i) Then f = True
Next i
If Not f Then s = s & IIf(s = "", "", ", ") & y(j)
Next j
End If
Difference = s
End Function