It's easy to agree that the Astra Zeneca vaccine is more stable. But this says nothing about it's efficacy.
So far, there is no statistically significant clinical trial data that tells us the efficacy of the Astra Zeneca vaccine. There is only data from much smaller groups of immunized people that suggests good efficacy, but not from large enough groups of people to allow useful, statistically valid, conclusions. The data presented to UK drug regulators for their approval of this vaccine has, to my knowledge, not been made available to the public. When Astra Zeneca completes it's US clinical trial, this data will be made available to the public as it is reviewed by the FDA.
Although there is some scientific data that suggests the Astra Zeneca vaccine can have similar efficacy as either the Pfizer or Moderna vaccines, there is no clinical data that directly answers the question – is the Astra Zeneca vaccine as effective as the Pfizer or Moderna vaccines? The only valid way to do this comparison would be in a randomized, double-blind, 3-arm clinical trial directly comparing these 3 vaccines in the same population of people. I doubt if it would be a good idea to perform this trial now during the pandemic emergency. Instead it would be wise to use all available vaccines, as much as possible, and as soon as possible. With the greater stability of the Astra Zeneca vaccine, plus the large manufacturing capacity in India, this third vaccine would make a valuable contribution.