We focus on QoE-optimized video delivery in a wireless heterogeneous network setting, where users equipped with multi-interface devices access a Dynamic Adaptive Streaming over HTTP (DASH) video service. We provide an Integer Linear Programming (ILP) formulation for the problem of optimal joint video quality and network interface selection and a heuristic algorithm to solve it, shown via simulation and testbed experiments to achieve near-optimal performance in terms of Quality of Experience (QoE), with reduced execution time. We further design a video delivery architecture based on the emerging Mobile Edge Computing (MEC) standard, where our video quality and network selection functionality is executed as a MEC application. Notably, our architecture transparently operates with standard DASH clients.