How hard is it for a Storyline user to switch to Captivate?

Rod gave you already a solid overview of the differences between Articulate StoryLine and Adobe Captivate, he has more experience with StoryLine, which I personally find too limiting. I may be wrong of course and it is a personal opinion. At the start StoryLine is indeed easier to use, but once you want to uplevel skills, I bumped on too many limitations. Some of my reasons:

• Lack of reusability, especially for interactions. I have a lot of libraries to use in any project (see  http://blog.lilybiri.com/dropdown-interaction-tips)
• Collaboration with other Adobe tools (Photoshop, Audition, Illustrator to mention only some)

Quite a while ago I wrote a blog, summarizing the main stumbling blocks for starting Captivate users. It is not a video, sorry. That blog is based on over a decade experiences in training and consultancy jobs, and answering questions daily in many places.

http://blog.lilybiri.com/challenges-for-starters

You could add to those challenges a fourth one: creation of responsive projects.

As a conclusion, it is not the tool which is important, it is the way that tool is used to create efficient, engaging eLearning assets. What you learned using StoryLine for that primary goal is of course not lost at all. Both tools can also lead to boring eLearning as well.

I have used both tools because as a contract e-learning designer I am often required to use whichever tool the client wants.

However, I used Captivate years before Storyline showed up on the scene so my viewpoint is tempered by the fact that I learned to use Captivate over a number of years when it really had no direct competitor.

Storyline's interface is almost a clone of PowerPoint.  Captivate's is entirely different.  You will likely find yourself very disoriented when you first try to understand Captivate.   Each Captivate CPTX can only be a single SCORM module as well.  There is no provision for dividing the module up the same way Storyline does with it's branching view.  Captivate also does not have the concept of Layers the same way that Storyline does.  You can still create them in Captivate but it's somewhat less intuitive.

Captivate's possibilities for creating interactivity are much more sophisticated than Storyline with several levels of complexity available (Actions, Advanced Actions, Conditional Advanced Actions, Shared Actions).  It also allows for more reusability of the actions you create.  Having said that, Storyline has a few tricks that Captivate could use when it comes to events that can trigger actions.  So, it depends on the kind of interactivity you want as to which you might prefer.  You will also find that Captivate is also much more JavaScript-friendly if you might be wanting to go that route to enhance your e-learning.

I believe Captivate to be the more powerful tool overall, but Storyline is easier to use because it is simpler.  Captivate support from Adobe is not as good as you may have experienced with Techsmith.  I know on their User Forum the Techsmith staffers are very active.  Here on the Captivate forum it is rare to hear from or get help from an Adobe staffer.  There are lots of very experienced people (other Captivate users) here to help you.  But Adobe is not well known for great support.

I strongly suggest you either sign up for a training course with a professional trainer or else binge on all theYoutube training out there.  Because there are just so many things you cannot possibly be expected to know about using Captivate, no matter how many other tools you have used for e-learning.