The answer varies depending on what metrics of sustainability are considered, and how one weighs them. 

In terms of environmental sustainability in materials or power usage, both are very similar. Although there is a much larger constellation of Android devices, an apples-to-apples comparison show that similar-sized devices use roughly the same battery technology, and are primarily made of glass, aluminum, steel, silicon, et al. For some of these materials, there may be additional concerns on where the base elements were mined, and under what conditions, which must be examined on a manufacturer-by-manufacturer basis. Apple, being a massive single company with a high degree of vertical integration, will generally have more visibility on their material sourcing.

For environmental sustainability of aftermarket parts and life cycle cost, Apple has hampered consumer repair efforts in the past to incentivize sales. It is tough to give a blanket assessment of Android here due to the variety of manufacturers. Some have similar practices, others are more consumer-friendly. This requires more direct comparisons between phone manufacturers at time of purchase, as this may change in the future.

Electricity usage between devices is primarily a function of the user, although Apple has recently introduced settings allowing users to limit charging when not hooked to a sustainably-powered electrical grid.

In the sustainability of the software systems themselves, it is likely that Android will have use cases in the future longer than Apple. Apple's source code is unavailable to the public, meaning that once Apple no longer exists there will be no further functional or security patches for existing Apple devices. Android, being partially open source, can be forked and recompiled for newer devices and patched as new issues are discovered by teams other than Google itself. This last point is held as very important by software engineers, but is usually not emphasized as much by the average consumer.
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