From Wikipedia:

The Q of a cesium fountain is about 10^10, or about 100 times higher than a traditional cesium beam. Although the resonance frequency is the same, the resonance width is much narrower (< 1 Hz), due to the longer observation times made possible by the combination of laser cooling and the fountain design. The combined frequency uncertainty of NIST-F1 is estimated as less than 5 x 10-16.

The Q of a rubidium oscillator is about 10^7. Undesirable shifts in the resonance frequency are due mainly to collisions of the rubidium atoms with other gas molecules and aging effects in the lamp system. These shifts limit the long-term stability. Stability is typically 1 x 10-11, and about 1 x 10-12 at one day. The frequency offset of a rubidium oscillator ranges from 5 x 10-10 to 5 x 10-12 after a warm-up period of a few minutes or hours, so they meet the accuracy requirements of most applications without adjustment.