less resource intensive becuase of the indexes, which are small databases which contain information about the users mail. Don't make your choice base on this alone, becuase it is not very easy to cluster cyrus servers. If you only have one server then choose cyrus, but if you possibly are able to accumulate older servers to cluster them, then choose courier.
So memory footprint and cpu consumption are not significantly different?
I gather then that to cluster cyrus, you need to not only share the actual mailboxes but also /var/imap?