Mark's Law of Rubidium Goodness... the bigger the box, the better it is. The HP5065A is one of the best units ever made. It can rival a cesium beam unit. The X72 is a horrid little creature. I would also go with the M100 / FRK units. The LPRO an FE56xx units were designed for telecom use and meet the specs required of them and low cost. The M100 and FRKs saw a lot of use in military systems. Which one you choose is best determined by your budget, applications, and requirements. A noisy little X72 could be just fine, or you may need the beastly HP5065.