I think it largely depends on what you’d like to achieve with your career and the kind of companies you want to be working with.
If you’re the hardcore development type, I’d look into getting certifications for all of the tools you’ve mentioned. I know a few people with marketing automation certifications across the board and they do really well as freelancers.
Also, look into which tools are in demand in your geographical area. Over here in Europe, Salesforce is huge and Eloqua is rapidly growing. My Salesforce certifications have certainly helped me secure a lot of well-paying work. Marketo, on the other hand, isn’t quite as big. I’ve heard of Adobe Campaign implementations, but I’ve never worked with it.
Pick a tool you know will get you work – and then get really good at it.
Furthermore, all of the tools you’ve mentioned are used by large companies. Exceptions are Salesforce Pardot and Marketo, which are also extensively used in the mid-market segment. If you just want to be developing, go for the big tools and specialize – it’s likely you can get work doing just development work for a larger corporation (freelance, too).
Smaller companies often need someone that can they can consult with on both marketing as well as technology. That’s where I operate and I love it. I’m certified in HubSpot and Pardot, which gets me exactly those kinds of projects.
I first answered this question on Quora: