As a patient hobbyist collector I try to buy one of each. My goal was to learn the history and theory of each one, explore the magnificent design, measure the actual timing accuracy, read as many old books or articles about them, and in some cases meet the designers of the clocks. Atomic timekeeping, like computing, is a new enough field that a few of the old timers are still around.

For the valley-mountain gravitational time dilation experiments that I've done I use 5071A cesium clocks. Even though 25+ years old, no commercial clock works better for this portable application. That is to say, rubidium clocks and older model cesium clocks are not quite good enough to detect these tiny relativistic effects with confidence.

Others here have mentioned prices. Surplus rubidium atomic clocks can be found for as low as $100 on eBay. Surplus cesium clocks range from $1k to $10k, depending on model and condition. Factory new 5071A are close to $100k so no one I knows is crazy enough to do that. If you are very patient and lucky on eBay you will find 5071A for as low at $2k to $5k once every couple of years.