I would point you more in the biology, physiology, or molecular biology direction. A big issue in the stem cell field (iPSC or otherwise) and for their application in drug discovery/disease modeling, tissue engineering, and/or cell-based therapy is to ensure you can differentiate the cells to the appropriate cell type. This really boils down to replicating the tissue- and organ-specific development context. Understanding how different cell types are formed and how they integrate into and function within specific types of tissue (brain, muscle, etc.) is crucial. Having a background in genetic engineering would certainly add high value as well. The bioengineering approach will most likely give you insight into the materials aspects of the stem cell field (extracellular matrix dis/advantages, 3D grafts and bioprinting, etc.) - certainly valuable as well. Take a look at Anthony Atala at Wake Forest, CIRM's website in CA, BlueRock Therapeutics, NovoHeart, and other industry groups active in the stem cell/tissue engineering space as well. And don't hesitate to send an email their way!