I had to capture an answer file for our x86 & x64 image. 
it seems that the answer file contains two lines that are different.
x86
Component-0=Program Files32
Component-1=RootLevel32
x64
Component-0=Program Files64
Component-1=RootLevel64
I am now using a vbs script to decide which answer file to use.
'-----------------------------------------------------------------------------------------------
' -- IS SYSTEM x86 or x64
'-----------------------------------------------------------------------------------------------
 DIM WSHELL, WSHELL_ENV, PROC_ARCH, OS_ARCH_TYPE
 SET WSHELL =  CreateObject("WScript.Shell")
 SET WSHELL_ENV = WSHELL.Environment("Process")
 
 PROC_ARCH = WSHELL_ENV("PROCESSOR_ARCHITECTURE") 
 
 If PROC_ARCH = "x86" Then    
  OS_ARCH_TYPE= WSHELL_ENV("PROCESSOR_ARCHITEW6432")
  If OS_ARCH_TYPE = ""  Then    
   OS_ARCH_TYPE = "x86"
  End if   
 Else    
  OS_ARCH_TYPE = "x64"    
 End If