The resolution you stream at heavily depends upon your computer's specs as well as your internet upload speed. Here's a little more information about each resolution setting (recommended upload speeds are based on streaming 1 camera and 1 window capture):
480p 30 fps - This is the recommended starting option for machines with lower specs. We recommend an upload speed of at least 3 Mb/s.
720p 30 fps (recommended) - This is the option we recommend for most of our users as it's an ideal bitrate for non-partnered streamers on Twitch. We recommend an upload speed of at least 5 Mb/s.
720p 60 fps - We only recommend using 720p 60 fps if you're a partnered streamer on Twitch or Mixer or are using Hitbox or YouTube to stream. This is a very high bitrate option so viewers with poor internet may run into buffering. We recommend an upload speed of at least 7 Mb/s.