Both offer long term employment at a good salary currently, but regional variations exist and they may shift with respect to each other over time as well. It's probably not a great idea to base such a major decision on expected remuneration. If you're just looking for money, become a plastic surgeon or a drug dealer.
If you're looking for a career that will bring you long term satisfaction, enjoyment, and fulfillment, then you need to look at which activities are more of interest to you and follow your passion. ABAP careers involve programming. If you love programming, then ABAP is the way to go. Basis work is system administration work. I wrote a blog on what kinds of things Basis folks do day to day which will give you more information about Basis.
You may also be interested in the following blogs:
I have written a blog and started a few conversations which touch on this directly:
There are a few blogs by other authors which I also think will be very helpful for you
I have a few more blogs which you might find interesting as well.
Hope this helps!