Either quad port adapter is suitable for iSCSI, but in a side-by-side comparison, you will notice some advantages to the specifications of the Intel® Ethernet Server Adapter I350-T4. The Intel Ethernet I350 server adapters are the newest, premium gigabit adapters offered by Intel. The Intel® Gigabit ET2 Quad Port Server Adapter was the best Intel Ethernet gigabit adapter for storage when it was launched, but the technology used is older and the power consumption will be higher.
If you have a tested system configuration using the Intel Gigabit ET2 Quad Port Server Adapter, I would say stick with the design that you know works. If you are planning a new system, then I would recommend the Intel Ethernet Server Adapter I350-T4. The newer adapter will give you a design with that includes the latest technology.
I hope this helps.
Added via edit: I should also have mentioned that if you anticipate moving to 10 Gigabit Ethernet in the near future, you might want to look at the Intel® Ethernet Server Adapter X520-T2. This adapter will work on your Gigabit network, but gives you the capability of moving to 10-gigabit and keeping the same adapter.