Exchange codes WRDS

[TorTheHammer]

Hey, I know this isn't a programming question and etc., but WRDS support uses so long time to answer me so I post it here as well. 

I've a problem regarding WRDS data with regards of the exchange codes. 

To make it short; 
I've a CRSP file and a Compustat file. 

Say that one permno/stock has an EXCHCD of 3(CRSP FILE) and HEXCD of 2 (In the CRSP file), which is different from each others - which one of those two should I use? I only want AMEX stocks so that I can merge it later with the Compustat file (Which for that permno has exchange code of 2). 

2 = AMEX
3 = NASDAQ