If Remarkety can be integrated with your website I would 10/10 recommend it every time. They have great reporting features and offer different options for automation campaigns or you can even create your own custom automation campaigns. If you are only using it for newsletters you'll still see benefits but it's not going to be near as beneficial unless you add in automation.
Smaato is very well suited for user acquisition and retargeting companies. Their overlap in terms of audiences and publishers with other exchanges is very limited, which makes it a great alternative when you need to scale a campaign and want to maintain your frequency cap. They work very well in branding campaigns as well as performance campaigns.
Smaato has a UI where you can control your line items on the traffic you want to receive. That is a key unique selling point that other SSPs don't offer. Most of the items that you will need like reporting can be done on a self serve manner and that is a big plus, specifically for changes that need to happen really fast and you don't need to deal with account managers that may take some time to answer emails.
Smaato's account management team is very professional. They are quite fast in terms of customer support, which is very helpful. One item that stands out is that they are willing to understand what you need for each campaign and what is the best way to move forward. They don't just jump quickly into trying to upsell more of Smaato's products (audiences, data) like other SSPs do.
Remarkety has many more features compared to Mailchimp. The integration to my webshop was extremely easy to set up and I like to use the interface from Remarkety. You get lots of predefined trigger points for your automation and building email templates is just as easy as it was with Mailchimp.