Hi,
That question is a bit hard to answer - it depends on your own taste, preferences & predilections.
Also, why are you restricting your choice to those two?
I suggest you read up on them both (SCN is a good place to start). See what fits.
ABAP covers a much broader area than PI. Once you know ABAP you can easily branch out into other areas like workflow, WebDynpro, etc.
Why not just do both? :-)
cheers
Paul