Pega is a comprehensive suite which offers a unique theme of BPM development in the market. A no-coding approach based on rules with inheritance makes Pega a very powerful product. However Pega, falls short on integration centric capabilities and very rigid to customize. On the other hand IBM comes with array of products which suits needs of varying degree. Advanced integration is solved by BPEL Process Server which has support for state based patterns and mediation. Dynamic rules and event management can be solved with WODM, Cloud to on-premise connectivity with Cast Iron, Enterprise gateway and security usecases with DataPower, Social BPM with IBM BPM , WODM, mobify with Worklight.
Pega has a little bit of eveything here and there. It solves the dynamic rule management, brings out the flavor of Social BPM and mobility with Antenna ( I guess) and predictive analytics as well in one single suite. There are certain usecases which needs to have a little bit of everything, however this little bits and pieces of functionality when its blows, Pega would have problems to scale. With IBM its a bit nightmare to maintain a variety of technologies, however you can wish to go for one without the other and go for something only when you truly need it.
Pega vs IBM
Its difficult to pick a winner. In nutshell when you want a full scale BPM with rich integration capabilities go for IBM BPM. On the other hand if you hava mature integration capability already, Pega can yield quick results for you as well. Pega's strength is its methodology. IBM BPM's strength is integration. Actually you can't go wrong with both in terms of implementation. My strong recommendation is to invest time to process analysis and pick a good vendor to support consulting and implementation.