I think it depends on your focus. If you are looking at specific vulnerabilities and real-world details, then World Engineer's answer is the way to go. From Information Security help center:
IT Security Stack Exchange is for Information Security professionals to discuss protecting assets from threats and
If you want to approach it from a more abstract angle of why one security model is more or less secure, in a computer-sciencey way, then Programmers would be better. But if you do ask here, it really needs to be more conceptual while also answerable. From our help center:
Programmers Stack Exchange is a Q&A site for professionals and students in software development and related fields who are interested
in getting expert answers on conceptual questions about software
Specifically, discussing the merits of each architecture would fall under "software architecture and design" in my opinion, as long as the question is worded correctly. It would have to be about a program you are writing (conceptual questions about software development) and not why Sun/Oracle did something a certain way (ask Sun or Oracle instead).