As an indicator, the following quoted from Mint 18 (based on U 16.04) release notes :
The 64-bit ISO can boot with BIOS or UEFI.
The 32-bit ISO can only boot with BIOS.
The 64-bit ISO is recommend for all modern computers (Almost all computers sold in the last 10 years are equipped with 64-bit processors).
ONLY an indicator - wattOS isn't Mint, nor Ubuntu