As you note they are the exact same Operating System.
The June 2003 release just means a lot less patching for you to get your system to a stable state.
If I were doing a new system, I'd probably go with the June release. I just did a system two months ago with the December 2002 release and it came out fine, though I had to do a major patch upgrade to get SD-UX past some performance problems.
Whether the install goes 32 or 64 depends on the hardware. If the hardware supports both, you'll get a choice. Most modern hardware is 64 bit only.
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