Merging Blog Networks

I have a couple small but established blog networks running WP 3.1.2 and BuddyPress 1.2.9 and I'd like to merge the administration and Buddypress social network stuff on a new network/domain while allowing the continued use of all secondary domains.

I see the 'Multi-Domains' and 'User Syncronization' plugins. Does one way scale than the other? I rather expect this to be a giant PITA to set up in any case, so I'm more concerned with implementing the smartest and most flexible long-term solution. If it matters, I will want to use the MultiDB solution when the traffic justifies the effort.

Can anyone recommend the best way for me to proceed?