I read MailChimp is to send bulk emails and Mandrill for transactional emails.
I am not so familiar with these terms. Can I have bulk transactional emails when I have a lot of users? or does bulk emails apply just when I sent one copy to many users?
My app is sending emails a few because is new, but it could grow. Will I be sending bulk emails when application grows? event if these emails are transactional?
What is a transactional email?
Transactional emails are typically non-marketing emails. Usually they are a thank you, confirmation, invoice, receipt etc. Something that is related to a transaction your customer made. General newsletters or promotional/sales emails (anything sent in bulk to a list) is not transactional.
Here is Mailchimp's What is transactional email link
Another thing to consider is how technical are you? Mandrill assumes you have a good bit of email knowledge and technical chops for design and sending. You can design an email in Mailchimp's editor and use it in Mandrill but even that isn't going to be easy without some pretty good understanding of the platform.
I looked at using Mandrill to send out a newsletter that needed more technical integration than Mailchimp could support and ultimately I scrapped the heavier tech pieces to use Mailchimp.
