Well, the 64 bit version supports more memory than the 32 bit version, which might be good to know..
I'ld choose the 64 bit version unless there is a specific reason to use the 32 bit version, if not else since Solaris 11 is 64-bit only, and if you have hardware which supports a 64 bit operating system, why not use it? The 64-bit support has been around since Solaris 7, and it doesn't affect performance..
If you have a 64 bit capable machine then I'd run 64 bits in the base machine. First, you mention Solaris 11 and if I recall correctly they dropped 32-bit kernel support in Solaris 11. So if you're running Solaris 11 it is a no brainer. Even on Solaris 10 I'd run 64 bits because you can take advantage of the larger address space 64-bits affords should you happen to need it. I don't see the down slide of running 64-bits. Also, if you plan to run a Solaris 11 guest you'll a 64-bit virtual machine because Solaris 11 doesn't support 32-bit kernels. I don't know for sure whether or not that requires an 64-bit host but that seems likely. So in my humble opinion, you're better off with the 64-bit host.