Amazon Athena and Redshift Spectrum are both large-scale data query services provided by AWS, which can execute SQL queries on data stored in S3. However, there are some key differences between them:

Similarities:

No ETL required: Both Athena and Redshift Spectrum can directly query data on Amazon S3 without the need for an ETL process.
SQL support: Both support querying data using SQL and are compatible with various data formats such as CSV, JSON, Parquet, etc.
Integration with AWS Glue: Both can integrate with the AWS Glue Data Catalog and use it as a metadata repository.
Pay-per-use: Both are billed based on the actual amount of data queried.

Differences:

Data warehouse: Athena is a standalone query service, while Redshift Spectrum is a component of the Amazon Redshift data warehouse. To use Redshift Spectrum, you need an Amazon Redshift cluster.
Performance: Redshift Spectrum generally has higher performance when dealing with large-scale datasets as it can leverage resources in the Redshift cluster to execute queries in parallel. In contrast, Athena is suitable for smaller-scale datasets and ad-hoc queries.
Permission management: Redshift Spectrum uses permission management in Amazon Redshift, which can support more complex permission control, while Athena uses AWS Identity and Access Management (IAM) for permission management.
Optimization: As Redshift Spectrum is part of Redshift, it can take advantage of Redshift's optimization features such as materialized views, partition pruning, etc., to improve query performance. Athena, on the other hand, requires manual optimization, such as appropriate file formats, partitioning, etc.
Cost: Athena's cost structure is relatively simple, billed based on the amount of data queried. The cost of Redshift Spectrum includes the cost of the data queried and the Redshift cluster.

Depending on your needs and business scenarios, you can choose the most suitable service. If you are already using Amazon Redshift and need to query data on S3, Redshift Spectrum is a good choice. If you only need a standalone query service to perform ad-hoc queries, Amazon Athena may be more suitable for you.