Ethernet is better, at this point in time.
First, 100Mbps is plenty. If your local media is encoded higher than 100Mbps, then you're doing something very wrong. Currently, BR allows only 48Mbps rates maximum for AV, and that is considered the gold standard right now. Even when the 4k standard on BR is finalized, those files will be 50-60Mbps. If your encodings are higher, you're using space needlessly with no added benefit. Saturating a 100Mbps connection just means it is buffering - transferring the video file from server to ATV faster than real-time - it doesn't mean 100Mbps isn't enough.
Second, LAN has far less interference, which means it has far lower chance of a dropped connection. With wifi, neighbors signals interfere, others users on the same network interfere, security systems and pretty much anything else using the 5ghz band will interfere. Heck, even minor solar flares and weird e/m from nearby powerlines can interfere with wifi. With ethernet, almost no interference.
Third, even 100Mbps LAN has better latency than 802.11AC (though AC latency is very good still). As gamers know, latency matters more than total throughput when it comes to maintaining a solid time-sensitive connection. The same applies to streaming.
When streaming, my goal is not to download a file as fast as possible; as long as it downloads faster than the time it takes to view it (faster than real-time), that is all I want in terms of speed. My goal is that the download is smooth, uninterrupted, and begins immediately. LAN provides that better than WiFi.
That said, I do hope they add gigabit in the next revision. While 100Mbps works 100% fine, I just feel like I'm wasting my gigabit LAN setup
Local streaming. Three ATV at home. Multiple concurrent users at home all accessing the Internet, local media, music media, streaming through the ATV's, iOS and Macs. That's why ATV4 is a great replacement for all your old ATV's at home. Using WiFi AC and not Ethernet.
More users doesn't tax the ATV more. Each ATV can only stream one thing at a time. More users taxes your server more. Not sure how ATV network speed is relevant to this.