i would like to be able to sync bookmarks and history for one device and just bookmarks for the second device
Over the past few years we've deliberately moved towards automatic, unattended, 'simple' sync. PICL might revisit these decisions and provide more configuration, but I wouldn't count on it. Consider also that Sync is not a backup service; if you're not syncing data to another machine, there is really no point syncing it.  <https://groups.google.com/forum/#!forum/mozilla.dev.identity>
well if i have 3 or more machines it still makes sense to allow a varying level of sharing. for example: one may not want the history on his work computer to be shared with the history on his home and mobile, but still have home and mobile share history between themselves, and bookmarks shared between all three. solving this seems like a simple matter of not sharing the "sync my" values themselves.