Afaik SB40 is not shared storage (without adding something else i.e. P4000 virtual appliance (according Product Bulletin: HP LeftHand P4000 VSA turns the SB40c into a scalable and robust iSCSI SAN, featuring, storage clustering for scalability, Network RAID for storage failover, thin provisioning, snapshots, remote replication, and cloning.). So i think X1800SB is better if two disks is enough for you (quorum lun for example). If you need bigger storage, you'll need both of them - X1800SB as iscsi gateway and SB40 attached to it as primary storage.
