Você deve instalar a versão de 32 bits (x86), a menos que precise trabalhar com planilhas ou documentos extremamente grandes.
Esta página da Microsoft explica porquê.
The 32-bit version of Office 2010 is the recommended option for most people, because it prevents potential compatibility issues with other 32-bit applications, specifically third-party add-ins that are available only for 32-bit operating systems.