Unless Twitch forces everyone to use Main which is why this was implemented in case they do once the HLS switch happens for real, stick with High. Luckily Twitch is currently transcoding your stream using Main profile. It is likely they want to avoid sending the transcoded stream to mobile, which is why they might make everyone stream in Main. Youtube live streaming also recommends Main profile, but they transcode everything anyways while still accepting High.
Main is easier to decode which is why its necessary for mobile devices. The quality will be worse if you happen to use Main profile for whatever reason. Once again this setting is in advanced so if you don't know what to do leave it at the default High unless they force everyone to use Main, and if that is the case Main shall become the new default setting most likely.