The use of RDMA makes higher throughput and lower latency possible than what is generally available through e.g. the TCP/IP communication protocol.
In other words, when an initiator writes data to a target, the target executes an RDMA read to fetch the data from the initiator and when a user issues a SCSI read command, the target sends an RDMA write to the initiator.
Bandwidth and latency of storage targets supporting the SRP or the iSER protocol should be similar.
On Linux, there are two SRP and two iSER storage target implementations available that run inside the kernel (SCST[5] and LIO) and an iSER storage target implementation that runs in user space (STGT).
This is probably because the RDMA communication overhead is lower for a component implemented in the Linux kernel than for a user space Linux process, and not because of protocol differences.