I think one element in the choice comes down to deciding if we want the default scope of these addresses to be global. Currently the FEC0::/10 prefix is called out in RFC3513 for special handling and everything else not listed (including FC00::/7) is to be treated as global unicast.
If we want these addresses to be handled differently from global unicast (like giving them preference over global unicast), then using the FEC0::/10 prefix is a good choice. If we want them to be treated the same as other global unicast than using FC00::/7 would be preferable.
Note that RFC3515 does allow for other specifications to define special handling for other prefixes, but as Alain Durand pointed out on the list doing so does put some additional burden on existing implementations.
Other opinions?
Bob
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