You ask a good question, but I think you are over-thinking this one. No one can truly anticipate all aspects of the market. I would not be comfortable with you taking my word that the SAP consulting option is the better one and choosing that. You just don't know what is going to be the most marketable area going forward. I think both of your career choices are appealing, and you need to make that decision based on where your passion and talent lies.
There's a big difference between the day-to-day job role of an SAP consultant versus being a project and delivery manager. I really think that you need to think about which type of responsibilities you would be more interested in mastering since they are so different. But stepping back from that, as a rule, it's always a risk to move from a managerial role back to a hands-on role. I don't think SAP is hot enough to accelerate you right back into management again.
If you move to hands-on, it may be a long time before you are back into a management capacity again. If you are truly torn between the two and like them both equally, I would vote for you to stay with the project management role. However, it sounds to me like your heart is more in this career change to SAP consultant. Just remember that being an SAP consultant is a very competitive undertaking, and as a new consultant you'll be up against many senior consultants.
To answer your question, I do believe that the SAP consulting will remain hot through the upgrade wave of the next three years, but as a relatively inexperienced consultant, you may find it more challenging to capitalize on that market than a more senior consultant would. There's no one right answer to your question, but I hope this discussion of the factors in your decision was helpful to you.