BuddyPress.org

This is my personal opinion, but If you’re building on top of an existing WordPress blog and theme, I would recommend WPMU so you can install BP on a secondary blog.

This is so you can separate the community side from your main WP blog. It will make things much easier from a theme perspective and for future upgrades to BP.

Plugins are a non-issue, unless you want to use the BP Group Wiki plugin or the BP Classifieds plugin.

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If you’re starting from scratch and you do not anticipate the need for user blogs, I recommend standard WordPress.