Should I use db_schema.xml or InstallData / UpgradeData scripts?

db_schema.xml, always, for table structure. The legacy InstallSchema / UpgradeSchema classes are deprecated since 2.3 and removed-in-spirit since 2.4. Declarative schema lets you diff via setup:db-declaration:generate-whitelist, supports rollback, and survives setup:upgrade --safe-mode. For data seeds (config rows, EAV attributes, sample products), use Setup/Patch/Data/<PatchName>.php implementing DataPatchInterface. Patches run once, are tracked in patch_list, and are idempotent if you write them right. Don't mix the two, declarative schema for structure, data patches for content.