Happy to tackle this one as I have both experiences to compare. Overall I would say there are more similarities than differences (dynamic project work in changing environments, the famous steep learning curve, lots of responsibility and strong personal development). However, I have noticed some key differences:

• Getting started on a project: In inhouse consulting, the fact that your clients are always a part of the same group means that we rarely start a project without having any prior knowledge of that area and the colleagues working there, and if we don't then someone else within ECON usually does and can share helpful insights or contacts. This really helps us hit the ground running and is not always the case in external consulting.
• Clients: In inhouse consulting, your clients are also your colleagues at the same time. In my experience this really helps building an open, trusting relationship from the start of the project. In our case many of our clients are actually former ECON consultants who know us and the way it works, giving an even smoother start.
• Project focus and length: Here external consulting can offer a broader scope of projects (which are usually also shorter - but at ECON you also get to see at least two projects per year), whereas inhouse has a much narrower focus - this could be a downside based on personal preference, but for many of us this is exactly while we go inhouse!
• Exit opportunities: In ECON at least, most colleagues go into jobs in the E.ON Group, whereas in an external consultancy the picture is much more diverse. If you already know you have a strong interest in a single industry, inhouse might be a great choice; if you are not sure or want to explore more industries, external might suit you better.