Daniel Eran Dilger provides an inside look into the 64-bit iOS 7 and ARMv8, and compares a possible 64-bit Android environment.
In short, the benefits of moving iOS apps to 64-bit include the
hardware advantages of the A7’s 64-bit cores (including more
registers, and likely more cache), the improvements and optimizations
inherent in the new 64-bit ARMv8 instruction set, and the requisite
API enhancements that come along with iOS 7.
The primary downside to the transition is additional system memory
consumption in cases where iPhone 5s users can’t transition all their
apps to 64-bit. This makes moving Apps Store titles to 64-bit a big
priority for Apple and, subsequently, a something it will push its
third party developers to support.