Your stream quality is going to depend hugely upon the stability and capacity of your broadband connection. Even faster pipes are often weighted heavily toward downloading rather than upload speed, so the broadcast quality you get out of any of these services is rather subjective. Streaming hosts generally ask you not to compensate for lagging with a variable bitrate stream, because this causes massive problems for some viewers, and higher bitrates mean that most viewers are forced to switch to a lower-quality transcoded stream.
If you happen to be able to fling packets at the Internet with limitless abandon, your selection of streaming service will, as ours does, come down to the maximum bitrate supported. Twitch offers a soft limit, so you can send whatever bitrate you like, but it would prefer you to keep your stream under 3,500kb/s, and it warns streamers who exceed this mark. YouTube Live, which transcodes every stream sent to it, can accept just about anything, but higher bitrates are squashed by its compression algorithm. Microsoft’s Mixer supports a maximum of 10,000kb/s without moaning, by which point your router will be on fire anyway.