As a social agency, there are a few things to consider when choosing a social management management system (SMMS) that may not be as important to an individual brand.
Platform support - While you may be primarily focused on a few platforms, video in particular, you want a SMMS that supports a wide range of platforms. For example, if it doesn't support Pinterest, it may be challenging to support B2C retail customers; if it doesn't support both LinkedIn pages and individual LinkedIn accounts, it's not good for B2B; if you're supporting local businesses, it should support posting to Google MyBusiness. If you lock into an SMMS with limited platform support, you limit your ability to support future clients.
Brand groupings - Unless you're willing to log in separately for each client (not out of the question), you want to make sure it's easy to pick all the channels for a client as a group, not have to manually pick them out of a long list.
Platform-specific posts - This is a pet peeve of mine, and many of the popular SMMS don't support this. You want to be able to send out the "same" message with slightly different versions on different platforms. For example, Instagram uses a lot of hashtags; Twitter might use one or two, if you have room, and you might also mention other Twitter users/accounts, and of course, there's a shorter length constraint on Twitter than other platforms. Some platforms support user mentions in Facebook and LinkedIn.
So the same post will vary slightly between platforms, but you want them to all go out at the same time; you don't want to have to compose and schedule them each separately. You also want to be able to aggregate metrics for the performance of the post across platforms.
These are just a few of the things to look at that you might not have thought of.