The Windows Azure Web Sites offers two options 32bits or 64bits. New sites come with 32bits by default. For a server configuration it feels a bit odd because, AFAIK, the vast majority of web servers have been in 64bits config for years now. Considering the usual use cases for ASP.NET sites, are there any reasons to favor 32bits over 64bits?
Depends on your workload.
IIS + .NET on x64 rocks and can support very large workloads. If you do load testing, you will see that x64 servers with sufficient CPU and RAM outperform x86 by a healthy measure.
If your workload is small, and/or you are going to load balance a number of servers, you may well find that x86 is fine. (I am a big believer in using smallest possible instances for the workload -- so much of IT is throwing metal at the problem, and it is wasteful. x86 is beautiful. Unfortunately, it is getting harder to get x86 instances.)
In our case, we load balance multiple x64 servers. However, during deploy operations, all or most of the load can be directed to a single server. In our case, if that server was x86, it would be stressed and very slow. As it is x64, it can handle the load and users don't notice.
Go 32 bit unless you need 64 bit address space - which is rare for a normal website.
The reason is simply that websites do normally not need the address space and 32 bi programs run faster.
Now, you say:
the vast majority of web servers have been in 64bits config for years now
The default setup for an IIS application pool is to start in 32 bit mode - for compatibility and performance reasons. The advice from Microsoft for IIS configuration is to use 32 bit application pools on a 64 bit server.
for the official statement.
You can be sure it will run on a 64 bit server, as there is no current 32 bit production offering from microsoft. But unless you need the higher amounts of RAM, the performacne impact is not worth it.