We used to have a rubidium and also a caesium standard when I worked in the cal lab.
From what I could gather , as stand-alone units, ther caesium standard was the better in the long term, but the rubidium was yhr\e more accurate in the short term.
That was, until the rubidium was connected to a GPS unit that updated it every day.
https://www.vectron.com/products/rubidium/rubidium_index.htm
https://spectracom.com/products-services/precision-timing/frequency-standards
Forum chat.
http://www.eham.net/ehamforum/smf/index.php?topic=66315.0
Interesting read on a super-accurate ytterbium clock
http://theconversation.com/sharper-gps-needs-even-more-accurate-atomic-clocks-38109