I would do both. As SAP progresses you'll need to keep jumping from one technology to another anyway.
I myself started with ABAP, then specialised in SAPscript, IDocs, HR, Workflow, TRM, CRM, PI, WebDynpro, and who-knows-what's-next. I try to stay au-courant in all of them.
If you restrict yourself to PI, you may be out of a job very soon.
So go ahead and buy those new shoes, but don't chuck out the old ones :-)
cheers
Paul B