Both Hyperledger Aries and Hyperledger Indy are platforms for building distributed ledger applications for identities.
What are the main differences between them? When to choose one over the other to implement a blockchain solution?
Aries is the agent (client) part of a decentralized identity (ledger, DIDs, verifiable credentials) application that is intended to be agnostic to the underlying ledger/DIDs/verifiable credentials layer.
Indy is a decentralized identity implementation including support for a ledger, DIDs and verifiable credentials.
Initial Aries work was to move the agent work in Indy to Aries, and so the first working versions of Aries use Indy underneath for the decentralized identity components. Over time, those components will become pluggable, and additional decentralized identity components will be supported. Thus, major parts of the indy-sdk will be deprecated, as they are implemented in Aries.
For building solutions, you should always be looking at Aries to start. You will need to know what Indy does, but not the details. The only time you would need to dive into Indy is if you what to extend its capabilities to support your use case.