You can use either virtual switch type for iSCSI.
I usually use a single standard vSwitch with an Active/Unused failover policy on the port group level. IMO this is an easy to implement, and maintain setup for dedicated iSCSI vmnics.
In case you do/can not physically separate storage (iSCSI) traffic, and other traffic types (e.g. vMotion, VM Network) you may be better of using distributed virtual switches with Network I/O control being activated.
André