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.
AdButler is suitable for any company size but I would said that it is better for small to medium-sized agencies. AdButler is highly scalable but the control panel/dashboard could be challenging to manage if you have many campaigns or many users. It's also a good alternative for ad networks, which would like to manage multiple relationships with publishers.
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.
AdButler is very convenient and very cost effective. Additionally, when you change a setup at the ad level, the server shows the change immediately; there's no delay. Ad Manager is better when you manage a significant amount of campaigns and users, but it's not that cheap in the end and you always have to wait for any change. EPOM is also a good solution, but the look and feel are old and it's complicated for some users to manage properly.