The verdict
If you do not yet send email to a list in a decent way, I'd pick Mailchimp first. Almost every small business needs that base, and nobody needs training to use it.
If you already have a list and a repeated demo or training that few people attend, pick eWebinar. A recorded session at a time that suits each person can lift attendance in a way another email will not.
At a small team scale the two often work side by side. Mailchimp promotes the session and follows up. eWebinar runs it. Check that registration and attendance data reach your email tool before you rely on that.