Cognos TM1 and Transformer are both OLAP tools used for data analysis and modeling, but they have key differences. TM1 is better suited for large data volumes, real-time reporting needs, and complex models that require on-the-fly calculations. It has faster performance due to in-memory processing and can handle write backs and merging large data sources. Transformer is better for consistent query performance with pre-aggregated cubes, but has limitations on cube size and does not support write backs or dynamic changes to models. TM1 should be chosen for large or changing datasets while Transformer is better for smaller, read-only analysis.