Advantages of 64bit version of Pivotal Web Server
The main advantage of 64bit HTTPD over 32bit is that it allows higher memory usage per process. 32bit processes generally cannot access more than 2GB of RAM. This could be necessary to allow resource intensive operation, such as massive image rendering.
Another significant advantage is the ability to read and create large files (larger than 2GB), which requires access to 64bit filesystem libraries.
Advantages of 32bit version of Pivotal Web Server
The main advantage of 32bit HTTPD over 64bit is compatibility with legacy libraries, as not all components that you may use are 64bit compatible (though this is increasingly less common).
Another advantage is efficiency. In general, 64bit runs a little slower than 32bit, depending on the usage. This is because everything operates with 8 byte pointers rather than 4 byte. More bytes are loaded/de-referenced for many operations, leading to higher CPU/RAM usage and slower execution.