Difference between attach and service request message is -
1. when UE is in EMM-DeRegistered state and want to move in EMM-Registered state then UE needs to perform attach procedure.
2. In EMM-Registered state UE can be in ECM-connected, RRC-connected state or ECM-Idle, RRC-Idle state. If UE is in ECM-idle, RRC-Idle state during EMM-Registered state and UE has data to send in UL/getting paging for MT data then UE sends service request to move in ECM-connected,RRC-connected mode.