Who each tool is really for
Copper
Copper positions itself as a CRM designed to work with Google Workspace while still being a real CRM: people, companies and opportunities as first-class records, with Gmail and Calendar tightly connected. It tends to suit relationship-led businesses such as agencies, consultancies and professional services, where accounts matter as much as individual deals and several people touch the same client over years.
Streak
Streak positions itself as a CRM that lives inside Gmail. Pipelines appear as views over your inbox, and the experience is closer to a very capable spreadsheet than to a traditional CRM console. It suits individuals and very small teams who want structure without leaving the mailbox, and who would rather add a column than configure an object.