When comparing Mandrill vs SendGrid, the Slant community recommends Mandrill for most people. In the question“What are the best transactional email services?” Mandrill is ranked 3rd while SendGrid is ranked 5th. The most important reason people chose Mandrill is:
Simply set your SMTP server to `smtp://user:API_KEY@smtp.mandrillapp.com:465` and you're good to go. There are libraries for using the API in a variety of languages, including Node.js.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Both SMTP and HTTP APIs are easy to setup
Simply set your SMTP server to smtp://user:API_KEY@smtp.mandrillapp.com:465
and you're good to go. There are libraries for using the API in a variety of languages, including Node.js.

Pro Clean UI
The UI is very clean and good-looking overall. While the Analytics may seem basic and not very detailed, the dashboard interface is very clean which also helps with the set up.
The left side of the page has the navigation to different parts of the dashboard. The main view displays all the most important data and analytics that you may need (emails sent, deliverability, reputation etc...). While the navigation itself is pretty straightforward and self-explanatory.
Pro Has sandbox mode for testing
Instead of sending real e-mails out, can use test keys which allows to test without actually sending out real e-mails. These test keys can also safely be shared inside the team since they cannot be used to send real e-mails on your behalf.

Pro Clear API documentation
The API documentation is very clear and makes it pretty easy to set up you application with Mandrill's API quickly and painlessly.
Pro Great mobile apps
Mandrill has companion apps for iOS and Android that allow monitoring the service using a smartphone.
Pro Highly customizable templates
Mandrill has made templates a core part of the product as they simplify the payload when compared to sending the full HTML each time and allow non-developers to design the template.
Mandrill uses the flexible content formatting of MailChimp templates. This paired with the API access to template management, gives the user a great deal of flexibility.
Pro Webhook Reporting
Mandrill provides and Event API also or "webhooks" through which it registers events when they occur. So when an activity occurs, the application server is called along with the activity that occurred. Events include opens, clicks, bounces, deliveries and spam complaints.
Pro User friendly rules engine
Mandrill allows setting up rules in a non-technical way.
Pro Great MailChimp integration
Mandrill is made by the same company that made MailChimp. As such, both products work very well with each other.

Pro Outgoing messages log
You can see the contents of all messages that Mandrill has sent out, for free for the last 24 hours, or for some cents per thousand messages if you need a longer retention period.
Pro Excellent analytics
SendGrid offers amazing analytics and reports. You can check which messages are delivered, which are marked as spam and much more. SendGrid also lets you download the data in order to look through it locally.
Pro Great support for beginners and experienced users
If you are just getting started with email marketing and transactional emails or if you get stuck somewhere, SendGrid includes amazing videos and guides to help you work out any problem you may have. What's more, it also has phone, chat and email support available 24/7.
Pro Both newsletter and transactional emails
Usually when choosing a service to send your application's emails you need two. One to send the newsletter or news and promotions from the marketing team and the other to send you application's automatic emails and messages.
With SendGrid you don't have to choose two because it doubles both as a transactional and marketing platform at once.
Pro Has a free tier
12,000 free e-mails per month.
Pro Easy to use API
The API is very easy to use and very intuitive. On top of that, the documentation for the API is complete and very useful, covering everything you need to know.
Cons
Con Has a bad history
When they decided not to support a free tier anymore they've set up a withdrawal period of only 2 months for existing customer before it stopped working (even for people who bought credits to send more than that free tier). Meaning people using it had a month to switch to another solution or start paying $20/month or more.

Con No free plan
There used to be a free plan which offered 12,000 free emails per month. Unfortunately it has now been discounted and there's no other free plan anymore. If you want to use Mandrill, you have to pay $20/month or more.

Con The documentation is split between two sites
The API is documented at https://mandrillapp.com/docs/, while howtos are in the Knowledge Base. Searching on the former for "Handlebars" doesn't find anything useful about using Handlebars.

Con Template rendering doesn't support Handlebars
The templates.render
call doesn't support Handlebars. This makes complex integrations more difficult. You can, however, use handlebars with messages.send
and messages.sendTemplate
.
Con Port 2525 is not encrypted
Most cloud providers block ports 25, 465, and 587. Port 2525 is commonly open but SendGrid only allows unencrypted messages via that port.
Note: There are also so some certificate issues (see https://support.sendgrid.com/hc/en-us/articles/200182008-Certificate-verification-failed-for-smtp-sendgrid-net)
Con Poor deliverability of emails
Con Not suitable for people without a technical background
There are some features which are missing by default from SendGrid (sign-up forms or follow-up messages just to name a few). In order to implement those in your emails, you need to know how to program because you have to code them yourself through SendGrid's API.
Con No sandbox/test mode
Does not support test keys that can be used to check integration with SendGrid and that cannot be used to send real e-mail out. It means that either you don't cannot verify that your integration with SendGrid works during development, or you have to use real credentials which should not be shared widely across your team.
Con Can't view contents of sent emails
They do not capture the sent emails. This means you cannot check that the email was sent correctly, and it makes it harder to debug issues that arise.
