IMHO it is not relevant which airline you get the ticket from. All RTW tickets within Skyteam have the same price independent of the ticketing airline and an AF-coded flight, for ex, will always book into the same AF booking class, independent of who issues the ticket. Your codeshare remark is still relevant, I think, but using codeshares is always possible no matter who issues the ticket. (Unless RTW tickets do not allow code-shares at all, in which case the ticketing airline would not be relevant either.)