This article presents a comparative study between Optical Burst Switching (OBS) and Optical Packet Switching (OPS) with variable-size packets on mono-service and multi-service network with Quality of Service (QoS) support In terms of performance criteria, loss rate in byte and access delay are considered. A 2-state Markov Modulated Poisson Process (MMPP) traffic generator is used A 14-nodes mesh network topology illustrating NSFNET and a ring topology are designed as the testbed. NS2 Network Simulation tool is used for our tests. According to the simulation results, we have seen that in both mono-service and multi-service networks. UPS is better than OBS under low traffic loads However, OBS performs better than UPS in mono-service networks and low priority service classes under high traffic loads. In this study, we have shown that choosing one of these two switching technologies (UPS or OBS) in the future depends on the traffic load and the topology of network, together with the considered service differentiation scheme. (C) 2009 Elsevier B.V. All rights reserved.