This paper tackles low delay adaptive video streaming over error-prone networks. Our framework consists of an encoding station, an edge server and a set of clients with various access rates. The edge server is capable of performing simple error concealment operations on the incoming data before forwarding the adapted media to its clients. We study two encoding scenarios: versions (multiple encodings at various output rates) and layers. We develop a unified end-to-end distortion model, which we use to derive the optimal coding strategy for both scenarios. Finally we analyze the performance of MPEG-4 coded versions against MPEG-4 FGS-coded layers in rate-constrained lossy environments. Experiments show that versions perform better than layers when the constraint on the aggregate rate is somewhat relaxed, for low to medium packet loss ratios.