Conservation and populace reduction is more sustainable environmentally, splurging is the reality and will probably remain so for decades/centuries to come, it has been the human standard for most of our recent existence across the majority of cultures.
Splurging encompasses what is so lovingly called development, which in reality is resource usage for the purposes of profit and the provisioning of an ever expanding population; as well as unfettered technophilia across all modes of life.
An universal one-child policy could create a world where conservation and development need not be at odds, with the enlargement of natural preserves as a consequence, though the likelihood of this in the near future is low.
Each human will be at odds when it comes to which one is better, and some, like me, will praise conservation while hypocritically continuing to reap the fruit of unsustainable development.