I was also in a dilemma a few years back in choosing whether BW on HANA or SAP BPC. I decided to go for BPC and I am happy that I took that decision.
Before jumping to full BPC work, I did a project on BW on HANA as well and enjoyed that too. But BW on HANA is kind of technical in nature where functional knowledge is good to have however not must. In BPC, if you wish to survive in the long-term you have got to possess business understanding (functional knowledge). This gives you a sharp cutting edge over BW-HANA and you stand out of the crowd.
I love BPC because I did few BW-IP (Integrate planning) projects in starting few years of my career and hence tasted what planning is!!
BPC is for planning/consolidation where you directly hit the business process and control it by budgeting/forecasting/consolidation. Hence it is one of the SAP tools which are used by high business officials (and in a few cases by CXO’s……mind you I am saying few cases) for improving enterprise performance. Hence the outcome needs to be of high quality and hence the demand of such people is quite high. For example BPC consolidation outputs are used for auditing/ statutory reporting and any mistake has the potential to screw up the company’s financial value. Hence you will see only CA or MBA’s finance doing BPC consolidation requirement gathering.
With BW-HANA you are preparing a decision-making tool for analysis to be used by managers/analysts. Without much functional understanding, I was still able to work in SAP BW, but slowly this space became overcrowded.
BW+BPC is a hot skill in the market and will continue to be so. With S4HANA, the demand is going to rise high because of IBP (Integrated business planning).
I would say go for BPC, but it all depends on your interest if you choose to do SAP BW training or SAP HANA training.