Thanks for contacting the Intel Embedded Community.
The main difference between the NHI350AM4 and NHI350AS4 is the Ethernet interface as you mentioned and is stated at the following information:
NHI350AM4 - 1 Gbs, 4 port 17x17
NHI350AS4 - 1 Gbs, 4-port SERDES
The NHI350AS4 was a SerDes only SKU introduced by marketing, because it has been discontinued as is stated in the Product Change Notification (PCN) 112221-00.
Please let us know if this information is useful to you.
If you’d like to see further specifications for the I350-AM4, you can check Intel’s product catalog at http://ark.intel.com/products/52966/Intel-Ethernet-Controller-I350-AM4#@specifications.
Have a nice day!