Display differences between 2 sheets

Hello, i wanna compare 2 sheets by column in differences (by matching text in cell and quantity of equal cells). E.g. in sheet "1" i have "1" "2" "3" "a" "a" "c" (each symbol have another cell), sheet "2" have "b" "c" "a" "2" "3" "1". I wanna see result "a" when i compare sheet "1" vs "2", and result "a" "b" when compare sheet "2" vs "1". And formula, which i type, sometimes works, sometimes no...What is wrong?
=IF('1'!A2<>"";IF(AND(SUMPRODUCT(--('1'!A2='2'!A$2:A$1500))<>0;SUMPRODUCT(--('1'!A2='2'!A$2:A$1500))=SUMPRODUCT(--('2'!A2='1'!A$2:A$1500)));"";'1'!A2);"")

Attached file