While Winshuttle is comparable, they are really two different tools in that Winshuttle is a hang-on tool while BPM is a catalyst tool. With WS, workflows start in SAP, flow to Winshuttle, and back in SAP (at least with my limited experience). In BPM, everything is housed in the BPM ecosystem, resulting in a much clearer business process flow.
If your organization is heavily invested in SAP as an enterprise wide solution already, BPM is really your only option. Solid, seamless integration, smooth workflow, and data trails for business analysis are impeccable. If you aren't already heavily invested, key questions to ask would be - Am I using too big a hammer? The results are impressive, no doubt, but the integration of BPM is demanding. A less-demanding, more intutive tool like Winshuttle or even AR Projekt that hooks in to SAP might be a simpler option.