For very small or startup companies, Mailchimp might be ok. But for companies that are focused on sales enablement, sales growth, better lead intelligence, and true marketing automation - HubSpot is simply a more sophisticated, robust tool. HubSpot's real value is in the reporting - the ability to see and track marketing ROI.
Mailchimp, in contrast, was not originally built for marketing automation - just to send emails. While they've since added the ability to create landing pages and workflows, the system is not intuitive and there are limitations with reporting.