If it's mandatory that you use WiFi, Roku claims the Stick+ has the best wireless reception. The power cord is actually the the WiFi antenna, so it should work better than the Ultra. But if it's ever possible that you could use a wired connection, then the Ultra is the only model available with an Ethernet connection. But both support 802.11ac for the fastest possible wireless connection.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.