An advantage of 64-bit Office is that larger file sizes are supported.
Also, workbooks of the 32-bit version of Excel 2010 can have «only» up to 255 sheets.
A 32-bit Excel installation can address only up to 2 GB memory. This might impact the SSAS Vertipaq Engine, which is used by Power Pivot, negatively.
Some disadvantages of 64-bit Office include:
Some features that are present in 32-bit Office are missing in 64-bit Office (notably: The legacy equation editor of Word and Word Add-in Libraries (WLLs)).
Rendering graphics might be subject to performance degradation because 64-bit does not support MMX.
ActiveX and COM add-ins that were written for 32-bit environments won't run in 64-bit Office. Similarly, In-place OLE activation might not work.
Access files with the extensions .mde, ade and .accde that have had their source code removed cannot be run on 64-bit Office
MAPI applications need to be rebuilt.
Because the disadvantages in most cases outweigh the advantages, Microsoft recommends for most users to use 32-bit Office.
A 64-bit version of Office runs on a 64-bit version of Windows only.
Microsoft does not support the installation of both a 64-bit and 32-bit version of Office on the same machine. Thus, in order to change the Office bitness, the «old» version needs to be uninstalled.
Some applications prevent the installation of a 64-bit Office. These applications include
The bitness of an Office installation is also revealed under File -> Account, then clicking on About ….
In Excel, the Worksheet functioninfo("osVersion") evaluates to a string like Windows (32-bit) NT 10.00 which allows to infer the bitness of the Office installation.