Crystal's ESR or Frequency Stability/Tolerance, which is more of a priority?
What "stability" means in this context is a bound on the effects of temperature. For example, the manufacturer may guarantee that the frequency will not change more than 10ppm over the range of -10 to +60°C. It is distinct from initial tolerance. 

ESR has an effect on the pullability of the crystal and the power dissipation and probably jitter.  

If you have too much power dissipation you can cause the crystal to drift over time just from that cause.

Personally I would be inclined to pick the one with lower ESR unless precise timing was of some great importance in the application. Or maybe the one with high availability, pedigreed supplier and lower price.