Alfalfa, by nature, is drought-tolerant.  When it gets dry, it will shut down, reserving its energy.  When it gets moisture, it will start back up again.  (But you already knew that.)

If my memory is correct, alfalfa is native to the SW Asia/ Middle East area of the world.  Think about the climate they have there, and think about the conditions it would take for alfalfa to survive.

In my opinion, if you wanted to look for characteristics that would help identify which alfalfa varieties are "better suited" for drought situations,  I would look at rooting depth.  My  opinion is that varieties will deeper roots will enable the plant to search for moisture which will help to keep it going.