The ATF600 actually came out in June 2015 with the WiFi controller, I bought my WiFi controller in late 2015 and used an ATF600 wall mount kit. (It even says it came out on June 2015 on the evohomeshop website, so not sure what they mean by "updated last year" ?)
The ATF300 is for the earlier non-WiFi models sold up until 2015. (Although some old stock was cleared for a while after this date)
Good luck with the new install, I hope it all goes smoothly.