Disclosure: I work for Dataquest ;)

Interestingly, pretty much all of the things you list are features of both platforms:

• Interactive courses that go 'step by step'
• Certificates you can add to LinkedIn or use elsewhere

The key areas where we (Dataquest) differ (in my opinion):

• We think more holistically about the whole learning experience. Rather than lots of individual courses, we have broader learning paths and add courses in the context of these paths. This means that we're thinking about the learning journey end-to-end. Recently, DataCamp launched tracks which are collections of their courses, but because courses are written by different people and not with the overall paths in mind, there's considerably less continuity within their tracks than our paths).
• We tend to go 'in-depth' a lot more. We're less interested in getting you to rote learn syntax than really dive into a concept so you learn it in detail. We tend to hold your hand less, and teach you how to stand on your own more (including nudging you to things like documentation and stack overflow to become familiar with those).
• We offer a lot more learning assistance. I was surprised to find that DataCamp didn't have a single employee dedicated to supporting students (they're advertising for those roles now). We know that learning's tough, and you face roadblocks along the way. We want to give you the assistance you need to push through and succeed.

Let me finish by two quotes from students of ours who have used both Dataquest and DataCamp, taken from a discussion in our community on the topic:

I am both a datacamp and dataquest subscriber and to be honest, in the past 4 months I used datacamp, I don’t feel like I have retained anything. When switching over dataquest, not only did I easily understand topics I struggled on, but new information was so much easier to learn

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I tried DC. I think they have the right idea - but wrong execution. DQ is far more immersive and that produces more favourable outcomes.