Hi Erik - the training period in Report Builder is fixed at 30 days with no option to change it.
Anomaly detection in RB currently functions similar to the Reports & Analytics UI, with a simpler approach to Anomaly Detection. Analysis Workspace uses more advanced statistical methods for detecting anomalies and takes into account holidays and YoY seasonality. It is the recommended tool for AD data. Read more here: Statistical Techniques used in Anomaly Detection
We hope to be able to offer the same logic used in Workspace in Report Builder at some point, but first will be working on updating our APIs to address these differences.
Hope that helps!